Béla Szeidl

Konkoly Observatory of the Hungarian Academy of Sciences
P.O. Box 67, H-1525 Budapest, Hungary

Introduction

It is now evident that at some level, the light output of stars, including our Sun, varies over different length of time. Little wonder that the investigation of variable stars has a special place in astronomy. Almost all of our knowledge of stellar interior come from the study of pulsating variables. The observed surface luminosity, radius and colour variations probe into the stellar interiors in much the same spirit as in terrestrial seismology. The fact, that the light curve parameters and periods of pulsating stars are closely related to their physical state and are well measurable renders them as excellent calibration objects.

The study of other types of variable stars has also large impact on our knowledge about the physics of stars. The eclipsing variables tell us information about the size and mass of the stars. The observations of variable young objects enlighten the process of birth of stars and draw picture on the interaction between stars and interstellar matter. The cataclysmic variables (dwarf novae and their violent relatives, the novae and supernovae) are clue objects in a number of astrophysical problems. Stellar activity manifests itself in luminosity variability in a broad spectral range from X-ray to radio wavelength. The rapid rotation and the convective atmosphere of the cool-surfaced stars render the operation of dynamo possible, producing a magnetic field that drives spot formation and other phenomena of stellar activity. Spotted stars allow to directly measure the stellar rotation, even the differential rotation.

The start of the systematic study of variable stars goes back to the mid-19th century when F. W. A. Argelander's epoch-making paper was published in Schumacher's Jahrbuch (1844). At that time only one and a half dozen stars of this type were known. This appeal for observing variable stars incited astronomers all over the world. By the end of the 19th century almost one thousand variables were already known mainly due to the rapid development of the photographic technique.

The light variation caused by eclipses of two stars orbiting each other was easy to explain as it was mainly an optical and mechanical problem. The understanding of the physics of intrinsic variables, however, proved to be more difficult. The pulsation theory was put forward as a hypothesis for discussion by H. Shapley^*

In the second half of the 19th century the observers in Hungary were rather interested in the objects of the Solar System, the planets and comets, so little wonder that records of star observations made here can hardly be met from this time. The first variable star observations in Hungary were made in 1879 by Friedrich Schwab, a German mechanician who worked for a while in our country.^*

Ó-Gyalla, after the nationalization

In 1899 Miklós Konkoly Thege handed over his private observatory at Ó-Gyalla to the Hungarian state. It was, indeed, a mile-stone in the history of the Hungarian astronomy. An institute run by the state had greater possibilities and a more prosperous future.

Konkoly Thege was keenly interested in the bodies of the Solar System, planets, comets, and meteors and was a passionate pioneer of photography and spectroscopy. Soon after the foundation of his private observatory regular sunspot observations had been performed. The change in the status of the observatory, however, necessitated the reconsideration of the scientific program and, in conformity with the scientific attitude of that time, a shifting of stress set in toward the study of the physics of stars. In view of the modest optical instruments available then at Ó-Gyalla, the photometric observation of variable stars was chosen as the principal program (Tass 1904b) but, of course, the traditional work had also been pursued. The 6 and 10 inch refractors of the observatory were used for the photometry. In 1901 and 1902 a wedge photometer manufactured in Töpfer's workshop in Potsdam was put to use for stellar brightness measurements. In order to eliminate the disturbing illumination of the scale of the wedge, its position was registered.^*

The variable star observations in Ó-Gyalla started on the evening of September 19, 1900. The permanent staff members of the observatory, baron Béla Harkányi, Antal Tass and Lajos Terkán took part in the ambitious program. Later Emil Czuczi and Zsigmond Fejes joined forces with the principal observers for a while. Harkányi, Tass, and Terkán selected the program stars very carefully. They consulted with S. C. Chandler's ``Third Catalogue of Variable Stars",^*

The unfavorable air conditions in the surrounding of Ó-Gyalla very often made the observations difficult. Three rivers, Zsitva, Nyitra, and Vág met near Ó-Gyalla, and the Danube streamed close to it. Due to the frequent fog it rarely happened that photometric observations could be carried out on some consecutive nights.

Between September 19, 1900 and January 2, 1903, 22 variables such as miras, {delta} Cephei type stars, eclipsing variables, and a nova were observed on 116 nights with the wedge photometers. During this period 425 observations were collected.

Between April 12, 1903 and November 16, 1908, the Zöllner-photometer was used for the photometry of variables. On the whole 1870 observations were obtained on 372 nights. 129 variable stars, mostly miras, red semi-regular stars, cepheids and eclipsing variables were on the list of observation. After 1908 only sporadic observations (5 nights in 1909, one night in 1910 and 8 nights in 1913) could be made, for the principal observers (A. Tass and L. Terkán) had other task, the photometric survey of the southern sky from 0^o to 15^o southern declination was undertaken, a continuation and extension of the ``Potsdam Durchmusterung'' to the South. This program was only partly executed, the visual photometry of all stars of the BD down to the magnitude 7.5 in the zone from 0^o to 10^o southern declination had been carried out and published (Tass & Terkán 1916).

The variable star observations were published in a series of papers (Tass 1904a, 1904b, 1905b, 1906, 1908a, 1908b, Terkán 1905). While observing the program stars two stars were suspected to be variable. In Cassiopeia, near T Cas, the 8th magnitude star BD+54^o49 (=190.1904 Cas) was one of the variable candidates (Tass 1905a, 1905c). J. G. Hagen,^*

Tass had hard luck with his other discovery, too. He noted that the star BD+22^o1576 in Gemini, close to the program star R Gem, varied in brightness (Tass 1905d) and it received later the variable star name TW Gem. The observations of C. Payne-Gaposchkin,^*

On February 21, 1901 14^h40^m GMT Th.D. Andersson^*

The nightly mean observations of GK Persei (Nova Per 1901) made at the Ógyalla observatory between 1901 February 28 and December 29 (filled circles). The nova, during its descending phase, showed fast, large light fluctuations. The observations of other observers before February 28 are also indicated (open circles).

Tass took part in the observations of another nova, too. Nova Aquilae 3 (=7.1918 Aql) was discovered on June 8, 1918 by L. Courvoisier.^*

One hundred years ago the explanation of light variation of stars was certainly known only for the class of eclipsing variables. Little wonder, that the staff of Ó-Gyalla Observatory of Konkoly's Foundation actively took part in the observations of bright eclipsing variables. Terkán (1906a, 1906b) analyzed his photometry of {beta} Lyrae obtained with the Zöllner photometer between July 14 and November 26, 1905 and determined the orbital elements of the system. This kind of investigation proved the applicability of the photometry for solving astrophysical problems, e.g. determining the size and mass of the stars.

At the very beginning of the 20th century, in line with the rapid development in physics (both experimental and theoretical) intensive investigations started around the world in order to get better insight into the problems of radiation and temperature (colour) of stars. At the same time, the use of photographic photometry just entered into astronomical application. This kind of investigations and works also started in Ó-Gyalla (Harkányi 1910, Terkán 1904, 1910). Terkán (1910, 1926) made an attempt at determining the colour and temperature variation of the eclipsing variables {beta} Lyrae and {beta} Persei.

The promising research was interrupted, the World War I broke out and the observatory got into a desperate situation. The comprehensive compilation of the variable star observations obtained at Ó-Gyalla could only be published later (Tass 1925). In consequence of the peace treaties, after the war Ó-Gyalla had belonged to Czechoslovakia, thus the staff left for and most of the instruments were transferred to Budapest. The Royal Hungarian Observatory of Konkoly's foundation found its new home on Svábhegy, a hill near Budapest.

The Start at Budapest-Svábhegy

In spite of the severe condition of the country, the Konkoly's Foundation reviewed in its new home near Budapest on Svábhegy, at an altitude of 470 m. The construction of the main building and domes was completed by 1929. The new 60 cm Newton-Cassegrain reflector was erected in a dome of 10 m diameter. The 20 cm Heyde refractor brought from Ó-Gyalla was housed in a 5 m dome and furnished with a new Graff-type wedge photometer. In 1932, the observation of bright stars using the Zöllner photometer attached to the 15 cm refractor that was also transferred from Ó-Gyalla to Budapest, was finished. Then, this telescope was dismantled and was replaced by a 17 cm Cook refractor from Kiskartal, from the private observatory of the Podmaniczky family. A 16 cm astrograph was fitted out to this telescope on purpose to start photographic photometry.

The scientific programs were essentially the same as those carried out at Ó-Gyalla. So, astrophotometry became the principal activity, with special emphasis on the photometry of variable stars. The regular observation of the Sun and shooting stars was, however, given up.

The visual observations of variable stars were already commenced with the Heyde refractor in 1929 and had been kept up as long as for 7 years, until 1936. Two ambitious astronomers, László Detre and Károly Lassovszky performed the variable star observations. Lassovszky took his doctor's degree at Pázmány University, Budapest in 1920 and then joined the staff of the reviving Konkoly Observatory. During time of the setting up of the observatory, he developed his knowledge at different North American and European astronomical institutes. Detre defended his doctoral dissertation at the Friedrich-Wilhelm University, Berlin in 1929 and, immediately after his returning, he started the research work at the observatory. Lassovszky took interest in eclipsing binaries in the first place, while Detre was keenly interested in pulsating variables. Nevertheless, both of them had either type of variables on their observing list. These years were very productive, a number of papers had been published mostly in the journal Astronomische Nachrichten. Detre and Lassovszky made the visual observations very carefully. Repeatedly, they checked the calibration of the wedge by measuring the stars of the North Polar Sequence or Pleiades. They found that the wedge-constant was not stable from night to night, therefore the sequence ``variable + comparison stars'' was chosen with great care. The linearity of the wedge, however, remained, which facilitated the reduction of the measurements. The main source of error of the Graff-type wedge photometer was, however, the inequality in the structure of the natural and artificial stellar images. The natural images heavily depended on the air conditions during the night (Lassovszky 1933c, Detre 1934c).

During the seven years, Detre and Lassovszky observed more than thirty stars visually and obtained more than 12000 observations. As mentioned Lassovszky was rather specialized in eclipsing binaries. He made extensive visual observations of four eclipsing variables. KR Cygni (Lassovszky 1936a) was observed on 81 nights and 1225 brightness estimates were obtained, while for AT Pegasi (Lassovszky 1935b) 1060 observations were collected during 59 nights. Similarly, for AB Persei (Lassovszky 1934) 718 visual observations were made on 115 nights, and for SV Tauri (Lassovszky 1938) 1260 observations during 100 nights. AT Peg, AB Per, and SV Tau are Algol type variables, while KR Cyg is of {beta} Lyrae type. Lassovszky made an attempt at deriving the orbital parameters, furthermore the size of these stars. Of course, these results are outdated, but his photometry is still of high value and is used for studying these stars' period changes. In 1931 Lassovszky (1931b) made a short scientific visit to Babelsberg Observatory and measured some variables exposed on Ernostar plates with the 40 cm astrograph, but the observations were scarce to do rigorous investigation. Nevertheless, he discovered a new variable (Lassovszky 1931a,c), that received the preliminary name 381.1931 And, later the final name AP Andromedae. Then he made a series of exposures on Ernostar plates with the 40 cm astrograph and was able to derive the type and the period of the new variable. It proved to be an Algol type eclipsing variable with a period of 1.59 days. The star was, however, too faint for the Heyde refractor at Svábhegy, Budapest, and it could not be observed visually (Lassovszky 1932a).

Detre observed visually only one eclipsing binary, SV Cam (Detre 1932c, 1933b). During 34 nights he obtained 489 visual observations with the Heyde refractor. Some forty years later this star turned out to be a very interesting object. It is not only an eclipsing binary, one of its components is an active, spotted star. The visual observations, as a matter of course, did not reveal this behaviour.

Over and above the eclipsing variables, Detre and Lassovszky had 6 RR Lyrae stars and 8 Cepheids on their list of visual photometry. Detre observed SW Andromedae (Detre 1934c) on 38 nights (550 observations), W Canum Venaticorum (Detre 1935b) during 55 nights (369 observations), RZ Cephei (only the times of light maximum were published, Detre 1931, Balázs & Detre 1938) on 39 nights (590 estimates), XZ Draconis on 20 nights (344 observations) and RU Piscium (Detre 1934a) on 37 nights (472 observations). Of these RR Lyrae stars only short discussions about the period changes were published, since in 1934 Detre and his co-worker, Júlia Balázs decided to carry out a more detailed and thorough investigation of RR Lyrae stars (see next chapter). Here I only mention one result that shows how accurate Detre's visual observations were: several observers^*

The visual observations of cepheids provided useful data to study the changes in periods of these stars. Both Detre and Lassovszky took part in the work and they collected 2515 observations for 8 cepheids. The program stars were YZ Aurigae (Detre 1935c, on 121 nights 239 observations), RW Cassiopeiae (Lassovszky 1932b, on 103 nights 520 observations), SZ Cassiopeiae (Detre 1933a, on 100 nights 344 observations), XY Cassiopeiae (Detre 1932a, on 87 nights 325 observations), SZ Cygni (Lassovszky 1933a, on 90 nights 265 observations), VY Cygni (Lassovszky 1933c, on 79 nights 240 observations), Z Lacertae (Lassovszky not published, on 93 nights 309 observations) and RR Lacertae (Lassovszky 1935a, on 92 nights 273 observations). Here only I mention two results. Lassovszky showed that the light curve variation of RW Cas suggested by M. Beyer^*

The accurate visual observations of Detre and Lassovszky proved the constancy of some stars that were supposed to be variable by previous observers. These stars are X Canum Venaticorum (Detre 1932b), TV Cygni (Lassovszky 1933b), TW Geminorum (Detre 1936b), TZ Herculis (Detre & Lassovszky 1934b, Detre 1936b) and UY Herculis (Detre & Lassovszky 1934b, Detre 1936b).

Among the program stars there were several stars classified as semi-regular variables. The visual observations of two of them were published. UZ Aurigae was observed on 146 nights and 350 observations were obtained (Detre 1934b, 1935a). UU Herculis was also intensively observed, on 135 nights 517 estimates were made (Detre & Lassovszky 1934a, Detre 1936b). Some 50 years later this star turned out to be a very interesting object with alternating periods and it has been recognised that this supergiant star is a representative of a new class of variables.

The summary of the visual photometry carried out with the wedge photometer on the 20 cm Heyde refractor of Konkoly Observatory is found in one of the observatory's publications (Detre & Lassovszky 1938).

Károly Móra was associate professor at the Astronomy Department of Pázmány University, Budapest and took active part in the work of the observatory. In the middle of thirties he joined the staff of the observatory and became its director in 1936. He published important papers on R Scuti, an RV Tau-type variable (Móra 1930, 1934). This star's variability was discovered by E. Pigott^*

Lassovszky, following the tradition of Ó-Gyalla, observed the bright Nova 605.1936 = CP Lac on 9 nights. The nova was discovered by A. V. Nielsen^*

The observations of Lassovszky were made on the nights June 20, 21, 22, 23, 25, 27, July 5, 6, and 7, with a Zeiss Petzval-7 cm Astrocamera on Eastman plates. During the two weeks of observation the photographic brightness of the nova dimmed from 2^m.40 to 5^m.92 (Lassovszky 1936b,c).

Detre's last visual observations were performed during a scientific visit in München (Detre 1942). Using the 10.5 inch telescope of the Universitäts-Sternwarte, München with a wedge photometer, he studied the brightness difference of binary components. Between the Right Ascensions 15^h and 0^h30^m and Declinations -8^o and +47^o, he measured 173 pairs from the Aitken's catalogue^*

The RR Lyrae program

At the end of the 19th century, a great number of short period variable stars were discovered in globular clusters. Soon the first field variable star of this type was also discovered by W. P. Fleming^*

In 1907 S. Blazhko^*

The striking changes in the maxima of the light curves of RR Lyrae that turned out to be periodic was first stated by H. Shapley^*

Apart from the discoveries mentioned above, several observers fancied that other RR Lyrae stars also had irregularities, non-repetitive features, changing light curves, short term or sudden period changes etc. As the observational accuracy left much to be desired, the observed and published irregularities were often doubtful. L. Detre strongly advised that the visual photometric observations were subject to significant systematic errors that could bring about ostensible light curve variations (Detre 1934c). Likely, the disputable irregularities stimulated Detre to conduct a systematic survey of ``short periodic cepheids'' and to scrutinize these objects. At that time, all pulsating stars with periods shorter than one day were called short period cepheids and no distinction was drawn between {delta} Scuti and RR Lyrae type stars. The very short periodic pulsating stars with period less than a quarter of a day, nowadays called HADS and SX Phe type stars also captivated Detre's interest.

In the thirties, the photographic photometry was the most simple technique that provided accurate observations exempt from systematic personal errors and could be carried out in small institutes with modest instruments. Therefore, Detre and his associate, Júlia Balázs (his wife to be) decided to execute a comprehensive photographic observing program of RR Lyrae stars. An astrograph fitted out with a photographic doublet of Zeiss (16 cm diameter, f/14) and a Rosenberg type micro-photometer were at their disposal. They tested different kinds of emulsions, measured the limiting magnitude at different exposure times and the field correction by measuring a number of stars in Selected Areas, North Polar Sequence, Pleiades and Coma Berenices. Their experience was that under favorable conditions 0^m.01-0^m.02 accuracy could be achieved, an accuracy that certainly hit the target set by themselves. The program and the results in general were described in several papers (e.g. Balázs 1963, Balázs & Detre 1961b, Detre 1957, 1960, 1965a, 1966a, 1967, 1970a, 1973).

The first variable photographed was XZ Cygni on July 20, 1934 and the last one VZ Herculis on July 8, 1958. During 24 years more than 50 variables were observed and about 60000 exposures were made on more than 3800 photographic plates. The exposition times were chosen between 30 sec and 5 min depending on the brightness of the stars. Several fellows, such as Imre Csada, Loránt Dezsö, István Földes, István Guman, Tibor Kolbenheyer, and others joined forces with J. Balázs and L. Detre for a short while.

Detre always made great efforts to improve the accuracy of observations. During the General Assembly of IAU held in Zürich, in 1948, he received an 1P21 photoelectric multiplier tube from H. Shapley as a present. The experiments with this tube were started in December 1, 1949 (Fig. 2). The photometer was attached to the 60 cm telescope at its Newtonian focus. The measurements revealed some defects of the equipment, the stability and linearity of the amplifier were not always granted. The troubles with the electronics seemed insolvable since during the post-war and cold war time good-quality electronic parts were not to be had in Hungary. In the long run, Th. Walraven helped to overcome the difficulties. He gave an amplifier to the observatory constructed by himself. Then the photoelectric photometry of variable stars had been carried out unbroken until the eighties, when the reconstruction of the telescope and dome was started. The 60 cm mirror of the telescope was first aluminized in 1962 and afterwards, the three-colour (UBV) observations were routinish running. Later, on the mountain station of the observatory a 50 cm Cassegrain telescope was put into operation. The photometer to this telescope was constructed at the observatory's workshop by Géza Virághalmy.

The start of the photoelectric observations at the Konkoly Observatory: First page of the log-book.

In the photoelectric observations almost all staff members of the observatory took part. The list of program stars of the photoelectric photometry was essentially the same as was chosen for photographic photometry in 1934.

In the course of observations, it promptly turned out that a variable was misclassified as an RR Lyrae star, or in reality, it was an eclipsing or a variable of an other type or a non-variable star. In this case, the observations and analysis had been published and the object was crossed out of the program.

The first object that was scrutinized within the scope of the RR Lyrae program was AV Pegasi. 217 observations were obtained during five nights in 1935 and neither light curve variations nor short term period changes were detected (Balázs 1935).

In the pulsation of VZ Herculis secular period change and light curve modulation were found by N. F. Florja.^*

The variability of RR Leonis was discovered in 1907. During the subsequent 30 years, a great number of both visual and photographic observations of the star were obtained. The different observers^*

BH Pegasi was also thought to be an RR Lyrae star with variable light curve by F. Lause.^*

DH Pegasi was observed on six nights in 1935 and 465 photographic observations were obtained. The star is of RRc type and has a stable character (Balázs 1937c).

The behaviour of AA Aquilae was also questionable. Both K. Bohlin^*

In order to study the very short period pulsating variable RV Arietis, a number of photographic observations were made. One of the stars used as comparison proved to be variable in brightness. The new variable 624.1936Ari discovered by Detre received the final name RW Ari, a typical RRc star with a period of 0.3543 day (Detre 1937a, 1937b).

RU Piscium is also an RRc type star. There were hints in the literature on the unusual period behaviour of the star. Between 1936-1942, on the whole, 759 photographic observations were made and analyzed by Loránt Dezsö (1945, 1949). He did not find any modulation in the light curve, only the period of RU Psc varied slowly. He suspected that this variation was periodic (with a period of three years and an amplitude of 0.0002 day). Later on, J. Tremko^*

Although the variability of VZ Pegasi was discovered in 1921, and was considered as an eclipsing variable, only in the fifties S. Kato^*

From his visual observations A. A. Batyrev^*

AT Andromedae represented a similar case. O. V. Tchumak^*

Those objects are also worth mentioning, which were erroneously classified as RR Lyrae stars and the Konkoly observations revealed their true nature.

The variability of WZ Cephei was discovered and mistakenly classified as an RRc star by H. Schneller.^*

WY Tauri was also classified as an RR Lyrae star by A. S. Williams,^*

The classification of ZZ Persei was also questionable. K. Nakamura^*

A. G. Lange^*

The Second Edition of GCVS (1958) classified both AT Herculis and BP Vulpeculae as RR Lyrae variables. AT Her was observed photoelectrically on 9 nights in 1960 but no light variation was detected. According to the spectrum, the star is of K0 V spectral type and is certainly not an RR Lyrae variable. The variability is questionable at all (Illés 1963). The photoelectric photometry of BP Vul was carried out during 11 nights in September and October 1959. It turned out that it is, in reality, an Algol type eclipsing binary (Illés 1960).

A summary of the photometry of RR Lyrae stars made at the Konkoly Observatory until 1956 was presented by Detre at the Variable Star Colloquium held in Budapest, 23-28 August, 1956 (Detre 1957). Likewise, Detre gave a review on the photoelectric observations of RR Lyrae stars with stable light curve carried out at the Konkoly Observatory at the first Bamberg Variable Star Colloquium (Detre 1960). Here, he gave an account of an interesting connection between the length of the stillstand on the rising branch and the amplitude of RR Lyrae stars.

At first, the photoelectric observations were made only in two colours, then from the beginning of the sixties in three colours. These observations made the investigation of the position of RR Lyrae stars in the colour-colour and colour-magnitude diagrams possible. Such an investigation was made for the variables of {omega} Centauri based on photographic photometry (Geyer & Szeidl 1965, 1970).

The Blazhko effect

Since S. Blazhko was the very first to notice the periodic oscillation in the phase of light maximum of an RR Lyrae star (namely of RW Draconis) we refer to this periodic variation as Blazhko effect. Later H. Shapley demonstrated that the phase oscillation was accompanied by the periodic variation in the shape of the light curve and in the height of maxima with the same period in RR Lyrae.

In truth, the investigation of the amplitude and phase modulation of RR Lyrae stars, the Blazhko effect, has brought in the international reputation of the Konkoly Observatory. The results of these investigations have been published in a series of papers.

The planned research and the preliminary list of program stars were described in one of the first publications (Balázs & Detre 1938). The first program star to be studied in detail, was RW Draconis. During 143 nights in the years 1936-1938, 1941, 1942, 1944, 1945, 1947-1952 taken as a whole, 7210 photographic observations were obtained at the Konkoly Observatory. The old visual observations from 1907 made the investigation of long term variations in both the fundamental and the Blazhko period possible. The changes in the characteristics of Blazhko effect could also be studied. The amplitudes of both the phase and the amplitude modulation were the largest in 1937, 0.085 day ~ 2 hours and 1^m.0, respectively. The amplitudes of modulation were much smaller after 1937, e.g. in 1941, 0.04 day ~ 1 hour and 0.5 magn., respectively. The striking change in the effect had to take place around 1938-1939, but that time no observations of the star were made at the Konkoly Observatory. Fortunately, observations of RW Dra were obtained at the Leiden Observatory in 1938. These unpublished observations were placed at J. Balázs and Detre's disposal and it became clear that the sudden decrease in the effect took place in 1938 (Fig. 3).

Making use of unpublished photoelectric observations, the changes in both the main and secondary periods could be investigated for a time span of 60 years. The very strong change in the amplitude of the effect in 1938 was accompanied by a very large sudden change in both periods. These variations were anticorrelated, the O-C diagrams were mirror images to each other.

The photographic observations obtained at the Konkoly Observatory also made the detection of a small amplitude 120 day period (almost three times of the conspicuous 41.6 day Blazhko period) possible. This longer period variation clearly came in sight when the Blazhko effect was stronger than average. The investigation of the long term variation in the phase modulation revealed a 7.4 year period, that could be barely noticed in the O-C diagrams of the pulsation and Blazhko periods (Balázs 1957, Balázs, & Detre 1938, 1952, 1962).

Sudden change in the Blazhko effect of RW Draconis in 1939 (after J. Balázs 1957).

Light curves (light surfaces) of AR Herculis at different Blazhko phases (after Balázs & Detre 1939).

The RR Lyrae stars with light curve modulation were usually observed only in a very limited phase interval of the pulsation period, during minimum, ascending branch and maximum light. J. Balázs and Detre carried out a comprehensive study of the Blazhko star AR Herculis: they covered the whole pulsation cycles at different phases of the secondary period, so thus they were able to construct the so-called light surfaces, the light curves at different fixed Blazhko phases. (More exactly, they gave the m({Phi}, {Psi}) brightness of the star in tabulated form, where {Phi} is the phase of the pulsation and {Psi} is the phase of the modulation.) In this way, the study of the Blazhko effect on the whole pulsation cycle became possible. These results aroused the interest of variable star researchers, even Leon Campbell and Luigi Jacchia spent one page expounding these results in their well-known book The Story of Variable Stars (The Harvard Books on Astronomy, eds.: H. Shapley and B. J. Bok, 1941).

The phase modulation of light maximum of the 0.470 day period RR Lyrae star, AR Herculis was first noted by S. Blazhko,^*

The amplitude of the light variation varied between 0.90 and 1.77 photographic magnitudes during the 31.5 day Blazhko cycle, while the amplitude of the time oscillation of light maximum was 0.06 day ~ one hour and a half (Fig. 4). During the modulation cycle the brightness of maximum was nearly the least when the momentary pulsation period was the longest (Balázs & Detre 1939).

Twenty years later Iván Almár repeated the investigation of AR Herculis on new extended photographic and photoelectric data sets. During the years 1946, 1948-1953, 1955-1957, 3511 photographic observations were obtained during 117 nights and, in the years 1958 and 1960, 1141 photoelectric measurements were made. These new data revealed that a 90.8 day period was also present (2.87 times longer than the 31.5 day Blazhko period). A comparison with the previous studies showed that in spite of the strong variations in the Blazhko effect during the decades the brightness of the brightest maximum in the modulation cycle practically had not changed and the form of the relation between the brightness and phase oscillation of light maximum was essentially the same for the different observing seasons. The amplitudes of the brightness and the phase oscillation of light maxima were subject to strong long term variations and between these variations no connection was found.

The new photometry and the published data made the investigation of the secular changes in both the fundamental and the modulation periods possible. Both of them exhibited perceptible changes: the pulsation period changed rather irregularly, the O-C diagram of the Blazhko period resembled a sine-like curve. Between the changes of the two periods no connection could be revealed (Almár 1961).

One of the most favorite Blazhko stars at the Konkoly Observatory was RR Lyrae itself. During the decades a large amount of observations of RR Lyr had been obtained by different observers at different sites, but the results were contradictory. H. Shapley^*

The light curve variation of RW Cancri was discovered by S. Blazhko^*

One of the most puzzling Blazhko stars is SW Andromedae. H. Shapley^*

In 1950 and 1953, 864 additional photoelectric observations were obtained on 22 nights. It was noticed that the length of the stillstand on the rising branch of SW And was changing with a period of 36.83 days, while the height of maximum varied 0^m.03 at the very most. This kind of light curve variation of SW And probably represents a specific type of Blazhko effect (Balázs & Detre 1954).

All the previous visual and photographic observers indicated that RV Ursae Majoris showed well-perceptible strong light curve variations, but were unable to establish the regularity of these variations.

In the years 1936, 1937, 1946-1952, 1229 photographic observations were obtained during 27 nights. From 1955 May to 1957 July RV UMa was observed photoelectrically on 35 nights and 1008 data were collected. The considerable part of the photoelectric observations were made in 1957, in all, 21 maxima were observed in that year, so thus the modulation period could easily be determined. The variable exhibited strong amplitude modulation, while the phase modulation of maximum was rather small. The variations could accurately be described by the combination of the 0.4680 day fundamental period with a secondary period of 90.1 days. The same periods were apparent in the earlier visual and photographic observations. Having used all the published observations, the O-C diagrams for both periods could be constructed. These diagrams contain cycles of the same length, but of opposite phase (Balázs & Detre 1957).

The investigation of RV Ursae Majoris proceeded at the Konkoly Observatory after 1957. In the years 1958, 1959, 1961-1965, 1968, and 1969, 5966 photoelectric observations (2378 in V, 2627 in B, and 961 in U band) were obtained on 150 nights.

As for the period changes the previous results were confirmed on a longer time base, the O-C diagrams of the pulsation and modulation periods run opposite in phase. During the Blazhko cycle there was no significant change in light curve in the phase interval 0.35-0.55. It was, however, a rather surprising discovery that, around phase 0.94, there was a point on the ascending branch that did not show any significant oscillation either (Kanyó 1976a).

Y Leonis Minoris was a known Blazhko variable. D. J. Martynov^*

The search for light curve modulated RR Lyrae stars and determination of the accurate Blazhko periods were also an important part of the program.

M. Beyer,^*

V. P. Tsessevich^*

The earlier photographic observations made at the Konkoly Observatory indicated that Z Canum Venaticorum had Blazhko effect. The star was photoelectrically measured at the observatory in 1964 and almost 500 observations were made on 12 nights. From the analysis of these data the Blazhko period proved to be 22.75 days, a surprisingly short secondary period for a star with a fairly long, 0.6538 day pulsation period. The height of the light maximum varied 0^m.45 in B and 0^m.38 in V during the modulation cycle, while the amplitude of the phase oscillation of maximum was 0.04 day ~ one hour (Kanyó 1966).

AR Serpentis was investigated by V. P. Tsessevich^*

W. S. Fitch, W. Z. Wisniewski, and H. L. Johnson^*

Inspired by a notice of L. J. Robinson^*

All the studies on Blazhko effect were focused on RRab stars. It was a question if RRc stars could also possess the effect. Detre observed TV Bootis, an RRc star, photoelectrically on several nights in 1955 and found that this star also showed light curve variation with a period of 33.5 days (Detre 1965b).

The globular clusters usually contain great number of RR Lyrae stars (e.g. Messier 3 contains over 200), therefore, their study is particularly suitable for determining statistical properties of Blazhko variables. It was concluded from the study of the variables in M3 that about 25-30% of the RRab stars showed the effect. A statistics of field RRab stars led a (less reliable) frequency of 15% (Szeidl 1965, 1973, 1976). As to the Blazhko stars in M3, another interesting observation was that for stars exhibiting variable light curves, the largest modulation amplitudes fitted the period-amplitude diagram valid for RRab stars with single period (Szeidl 1965). A definite negative correlation was found between the noise of pulsation period and the length of the Blazhko period (Kanyó 1976b).

During the years considerable knowledge accumulated at the Konkoly Observatory on the RR Lyrae stars with Blazhko effect. From time to time, it was summarized and published in review papers (Detre 1954, 1956a, 1957, 1962a, Szeidl 1976). In one of the reviews, Detre made an interesting observation. He arranged H. W. Babcock's^*

Júlia Balázs propounded the oblique pulsator model as an explanation for Blazhko effect almost fifty years ago (Balázs 1960). If the magnetic axis inclines to the rotation axis of the pulsating star, then, depending on the geometry and the strength of the magnetic field, the Blazhko effect may have a natural explanation. If this hypothesis holds, then the Blazhko period equals to the rotation period. During the years, competitive theories have been advanced but we still lack for understanding of the Blazhko effect.

The situation became more complicated, when the 4-year cycle of RR Lyrae was discovered. After discussing and publishing their early photographic observations of RR Lyrae (Balázs & Detre 1943) its observation was kept on photographically, then photoelectrically from 1950. It seemed that the intensity of Blazhko effect had irregularly changed during the years. Detre wanted to mention it as an example for non-periodic effects in his introductory talk at the IAU Colloquium ``Non-periodic Phenomena in Variable Stars'' (Detre 1969c).

While preparing the figure of phase and amplitude modulation of RR Lyrae for different years by the author of the present review at Detre's request, it became conspicuous that the effect almost disappeared in the years 1963 and 1967, suggesting that the Blazhko effect might have a 4-year cycle. A very intensive observation of the star was started in 1969 and the transition between the new and old cycles was observed in 1971. At the end of the old 4-year cycle the phase variations during the 41-day Blazhko cycle died down almost completely and then, the new cycle started with a rapid increase of the amplitude. The amplitude of the maximum-variations was only 0^m.07 at the end of the old cycle, and then very rapidly became as large as 0^m.16 during 1971 and increased to 0^m.27 in 1972. The beginning of the new cycle was accompanied by a phase shift of 10 days, about a quarter in the phase of the 41-day Blazhko cycle (Fig. 5).

A whole 4-year cycle of RR Lyrae (after Szeidl 1976).

The Konkoly Observatory had almost forty thousand unpublished observations of RR Lyrae in 1972. Supplemented by others' photoelectric observations (1947: Th. Walraven,^*

These results were fully confirmed by new observations, the transition from the old to the new cycle was also observed in 1975, and the phase discontinuity in the 41-day period was again about 10 days (Szeidl 1976).

The identification of the 4-year cycle with magnetic cycle was very tempting and later it become the subject of heated debate.

Period Changes

W. Ch. Martin's^*

In 1937 L. Detre commenced the study of RR Lyrae stars of globular clusters M3 and M15 at the Konkoly Observatory. Later M. Lovas started the observation of RR Lyrae stars in M5 in 1951, and M56 and M92 were also included into the program. During the years 1937-1966 more than one thousand photographic plates were made on these clusters with the 60 cm Newton telescope. The plates were taken by Júlia Balázs, Katalin Barlai, L. Detre, G. Kulin, M. Lovas, I. Ozsváth, and B. Szeidl.

I. Izsák started investigating the period changes of the RR Lyrae stars in M15. The preliminary results were presented in the 1956 Budapest variable star conference (Izsák 1957). In the course of this study three new variable stars were discovered in M15 (Izsák 1952).

Between 1937-1951 115 plates of M56 were taken with 20 minutes exposure time. Júlia Balázs started analyzing the variables in this cluster. Comparing eight pairs of plates, she discovered two new variables (Balázs 1952a).

The study of period changes of RR Lyrae stars in M3 was started by I. Ozsváth. A short account on the preliminary results was given also at the 1956 variable star meeting (Ozsváth 1957).

For the final investigation of the variables of M3, 214 plates obtained at Budapest and 17 plates supplied by the Hamburg Observatory were used. 121 variables were measured with the microphotometers of the Konkoly Observatory and 13 variables (that could not be measured owing to crowding effect) were estimated. Out of the 134 objects measured, there were one red semiregular variable and one W UMa type eclipsing binary (RV CVn), a foreground star. As a whole, O-C diagrams of 125 RR Lyrae stars have been constructed, for most of them (116) for almost a 70 years timebase. Considering only the variables observed since Bailey's discoveries, the O-C diagrams could be fitted by a straight line for 8, by a positive parabola for 23 and by a negative parabola for 25 variables. No systematic trend was found in the direction of period changes. About half of the O-C diagrams could not be approximated by quadratic formulae. As a rule, the variables with secondary period had very complicated O-C diagrams. It was also an interesting observation that several RR Lyrae stars (both RRab an RRc) had sine-like O-C diagrams.

A review on the period changes of RR Lyrae stars in globular clusters was given by Szeidl at the IAU Symposium on Variable Stars and Stellar Evolution held in Moscow in 1974. There were doubts about the character of the changes, whether the periods were relatively constant (apart from slow secular evolutionary changes) for long time intervals followed by brief intervals of spontaneous, abrupt changes or, whether the variations were relatively smooth. Two field RR Lyrae stars which were on the Konkoly Observatory's program provided examples for both cases. The period of RR Leonis (Balázs & Detre 1949) had increased smoothly, but the rate of change was not at all constant. On the other hand, the period of RR Geminorum changed suddenly around JD 2428900 within a brief interval. It was originally hoped that the period variations would depend on the positions in the colour-magnitude diagram, this expectation, however, was not fulfilled. We could not find any connection between the position of a star and its period change either in M3 or in M15. The final conclusion was that it was impossible to attach any evolutionary significance to the observed period change of an RR Lyrae star. Likely, {omega} Centauri is the only exception, in which some evidence for evolution effects may be present (Szeidl 1975).

L. Detre made an attempt at investigating the period changes of field RR Lyrae stars, but the sample was too small for drawing statistically significant conclusion (Detre 1955, 1970b). These studies, however, showed that, over and above evolutionary changes, some kinds of other effects exerted significant influence on the period causing period changes of different amount and of different direction.

Júlia Balázs and L. Detre called attention to mechanisms which were not connected closely with the physics of the star and could produce apparent period changes. If a variable moves as compared with the observers (even if in straight line with constant velocity) its observed period changes in consequence of the Doppler effect. In the case of constant velocity, the period is apparently increasing (Detre 1969b, 1970b). In a paper on period changes in variables and evolutionary paths in the Hertzsprung-Russell diagram Júlia Balázs and L. Detre applied the theory of random walk to the O-C diagrams of pulsating variable stars. If the period has random fluctuations, noise with {sigma} standard deviation, then the O-C diagrams are random walks due to the cumulative nature of the fluctuations and do not represent real period changes. It was found that {sigma} was an important parameter of variable stars and correlated with the rapidity of evolution (Balázs & Detre 1965).

Other pulsating variables

- {delta} Scuti variables

In the early days (in the thirties) {delta} Scuti stars were not distinguished from RR Lyrae stars, so it is not a wonder that they were also on the observing list at the Konkoly Observatory.

Immediately after the discovery of the variability of 391.1934 Aqr = CY Aquarii, its photographic photometry was commenced by J. Balázs and Detre. During nine nights in 1934, almost 200 observations were obtained and the elements of the light variation were derived. Nine maxima were observed that pointed to single periodic nature of the star (Balázs & Detre 1934, 1935). Later, it was observed photoelectrically that confirmed the previous results (Detre & Chang 1960).

Previous observers of XX Cygni referred to its light curve variations. Analyzing more than 300 photographic observations obtained in 1936, no sign of secondary periodicity was found (Detre 1936a).

Based on his visual observations, A. G. Lange^*

In the thirties, the question of multimode stellar pulsation aroused the interest of the variable star astronomers and captivated Detre's interest, too. He analyzed the photoelectric data set of {delta} Scuti obtained by E. A. Fath^*

At the Konkoly Observatory the observation of {delta} Scuti stars has proceeded forth photoelectrically (Detre 1957). I. Guman (independently from W. Fitch) discovered that VZ Cancri was also a double mode variable. He observed the star photoelectrically between April, 1951 and February 1954, and collected more than 1200 observations on 24 nights. The measurements were made without filter. The light curve variation and the differences in height of the 26 observed maxima were conspicuous. According to both the periods and the light variations the star behaved resembling AI Velorum and SX Phoenicis (Guman 1955a).

The variability of SZ Lyncis was discovered by C. Hoffmeister^*

V. P. Tsesevich^*

- AC Andromedae

Soon after P. Guthnick and R. Prager^*

Little wonder, that the strange behavior of this star aroused L. Detre's interest and in 1935 he included AC And into the observing program of the Konkoly Observatory. Between 1935 and 1954, 5670 photographic observations were obtained that were later analyzed by I. Guman.

In order to get a real picture about the nature of the light variation of AC And, the star's intensive photoelectric photometry started in 1958 at the observatory. For the year 1962 Detre organized a campaign, too (Detre 1962b). In the long run, AC And was observed photoelectrically on a total on 78 nights in the years 1958, 1960, 1961, and 1962 and more than 10000 observations were secured in the Johnson U, B, and V bands. The analysis was, however, restricted to the 4662 yellow magnitudes, and revealed that AC And had its fundamental and first and second overtone radial pulsation modes, all excited and nonlinearly coupled. This variable star was found to have a mass of M=3.1M_Sun and to be a high-mass analog of the {delta} Scuti stars, in the hydrogen-shell-burning and helium-core-contraction stage of evolution (Fitch & Szeidl 1976).

- Cepheids

In order to test the photoelectric photometer, L. Detre observed bright cepheids, such as FF Aql, {eta} Aql, SU Cas, TU Cas, {delta} Cep, X Cyg, SU Cyg, S Sge, and T Vul, but these observations have not been published.

The long period cepheid CY Cas was investigated on Moscow archive plates and its period was corrected (Almár 1959).

The beat period cepheid TU Cassiopeiae was intensively observed by Erzsébet Illés in 1960-62 and by L. Szabados after 1971. The light-surface at different beat phases was constructed and the variation of the periods was studied (Illés 1968, Illés & Szabados 1976).

The Observatory's new photometer on the 50 cm Cassegrain telescope was put into operation in 1972. The instrument equipped with a UBV photometer made the expansion of the observatory's program possible. The observations of northern cepheids brighter than 12^m were started in 1972. One of the purposes of this observational program was the search for beat cepheids. One new double mode cepheid was discovered: BQ Serpentis had two periods, P₀ = 4.271 days fundamental and P₁ = 3.012 days first overtone, and P₁ / P₀ = 0.705 period ratio (Szabados 1976a). Some results of the survey as by products were achieved right away. V445 Cas was wrongly classified as a cepheid, it is in reality an eclipsing variable of {beta} Lyrae type (Szabados 1974a). V361 Per was catalogued as probable cepheid. It turned out to be an early type irregular variable (Szabados 1974b). The Bamberg variable BC Dra was assigned to cepheid. In truth, the star is an RR Lyrae variable of long (0.720 day) period (Szabados et al. 1976). J.D. Fernie and J.C. Hube^*

In 1966, S. Demers and J. D. Fernie^*

- {beta} Cephei type

RS Sextantis was identified as ``related to the {beta} Canis Majoris stars'' by A.B. Underhill.^*

- Red variables

On the plates taken for the RR Lyrae program, variable stars of other types may be found and could be investigated. BT Lyrae appeared on the plates (close to their edge) taken on the globular cluster M56 and its measurements showed that there were departures from regularities and the form of successive maxima and minima strongly varied. The period could be corrected to P = 167.85 days (Balázs 1952a). ST Draconis is near RW Dra. E. Hartwig^*

On the plates taken for the study of AC And, two other variables could be found and measured. AI Andromedae is a Mira type variable and showed strong period variation (Guman 1952, 1955b). BE Andromedae is an M6 type semiregular variable. Its average period was newly determined, 157 days as against the previous value of 137 days, but the actual periods showed large fluctuations (Guman 1955b).

- Cataclysmic variables and flare stars

Nova Herculis 1963 was discovered by E. Dahlgren^*

One of the light outbursts of the very interesting object Rosino-Zwicky near M88 was detected on plates taken by the observatory's 60/90 cm Schmidt telescope for the supernova search program. The object brightened by 6 magnitudes from the middle of March to the beginning of April 1965 (Lovas 1965). In the course of the flare statistics program, a new U Geminorum star was found in Cancer (Jankovics 1973).

The observatory took part in the photoelectric monitoring of flare stars. AD Leonis was observed and events were recorded (Szeidl 1969a, Barlai et al. 1972).

- Binary stars

Although the observatory's main program was the study of intrinsic variables, some eclipsing and spectroscopic binaries were also investigated.

On the plates made for studying AV Vul, the {beta} Lyrae type variable, CD Vulpeculae could also be measured. The amplitude of the primary minimum was about 1^m, and that of the secondary minimum about 0^m.4 (Guman 1951).

The long period eclipsing binaries Z Aurigae and 32 Cygni were photoelectrically measured (Detre & Herczeg 1952, Herczeg 1956a, 1956b).

The orbits of visual and spectroscopic binaries were also discussed in several papers (Batten & Szeidl 1972, Herczeg 1957a, 1957b).

In 1959, K. K. Kwee^*

Attempt was made to interpret the O-C diagrams of four eclipsing binaries (W Delphini, Z Draconis, TX Herculis, and RV Lyrae) with the effect of a hypothetical third body, and the mass functions and orbital elements were determined (Illés & Almár 1963a,b)

Putting into operation the 50 cm Cassegrain telescope, more telescope time was at disposal of the staff and the starting of new programs become possible.

The regular photoelectric observations of minima of eclipsing variables were commenced in 1973 (Patkós 1975, 1976).

Epilogue

In this review I made an attempt to summarize the results of variable star research at the Konkoly Observatory published until 1976. This date was chosen for the simple reason as I tried to stress the results and achievements ``in which László Detre had a hand". Of course, there were research works commenced before 1976 and concluded and published later on. Here I quote only some examples.

The old photographic observations of AC And were measured and analyzed.^*

The period changes of single mode high amplitude {delta} Scuti stars (CY Aqr, YZ Boo, XX Cyg, DY Her, EH Lib, SZ Lyn, and DY Peg) were investigated also making use of the old photographic and photoelectric observations of the observatory.^*

The behaviour of the peculiar W Vir type star, RU Cam had been followed from 1966 during 16 years. The unique data set covered almost continuously the light variation for that time interval.^*

In the frame of the photographic observational program of RR Lyrae stars in globular clusters, a great number of plates were taken with the 60 cm telescope on different clusters from 1937 up to 1966. Measurements of variables in M15 were accomplished,^*

In this review I have not alluded to two subjects, although both belong to the field of variable stars. In order to make the most the 60/90 cm Schmidt telescope, L. Detre initiated the search for extragalactic supernovae (Detre 1974) and for flare stars in galactic clusters.^*

The variable star research at the Konkoly Observatory had new perspective when the development of its mountain station was completed in 1975. The new 1 m RCC telescope was right away furnished with an uncooled UBV, and later on with a refrigerated photon-counting UBV(RI)_C photometer,^*

The variable star research at the Konkoly Observatory well-founded by L. Detre during his life has always had international reputation and the variable star community of the observatory has always taken active part in the international co-operation. Since 1961 the Information Bulletin of Variable Stars, the official publication of the IAU Commissions 27 and 42 has been issued by the observatory. The editors and co-editors were/are L. Detre (1961-1974), B. Szeidl (1968-1990), L. Szabados (1983-2000), Katalin Oláh (1990-) and Johanna Jurcsik (2000-). L. Detre and B. Szeidl fulfilled the duty of presidency of the IAU Variable Star Commission in the years 1967-1970 and 1985-1988, respectively. The observatory organized three international conferences on variable stars before 1976:

• Konferenz über veränderliche Sterne, Budapest, 23-28 August 1956
• Non-periodic Phenomena in Variable Stars, IVth Colloquium on Variable Stars, Budapest, 5-9 September 1968
• Multiple Periodic Variable Stars, IAU Colloquium No.29, Budapest, 1-5 September 1975

In this short review I essayed to outline the first 75 years history of variable star research carried out at the Konkoly Observatory. Although the circumstances were not always favourable for research work, the accomplishment was impressive and has inspired, and may inspire the succeeding generations. The publications of staff members of the Konkoly Observatory on variable stars between 1901 and 1976 are given in the references section.

Acknowledgements The author is very grateful to Mr. Mihály Váradi for helping in the preparation of this paper. The financial support of OTKA grants T-043504, T-046207 and T-048961 is acknowledged.

References

Aitken, R.G., 1932, New General Catalogue of Double Stars within 120deg of the North Pole. 1-2., Carnegie Inst. Washington

Allen, L.B., Marsh, F.F. 1932, Harvard Bull. No. 888

Almár, I., Illés, E., 1966, Mitt. Sternwarte Budapest, 60, 1 CoKon60

Almár, I., 1959, Peremennye Zvezdy, 12, 437 (1959PZ.....12..437A)

Almár, I., 1961, Mitt. Sternwarte Budapest, 51, 1 CoKon51

Almár, I., 1968, Inf. Bull. Var. Stars, 260, 1 (IBVS N°.260)

Andersson, Th.D., 1901, Astron. Nachr., 154, 363

Babcock, H.W., 1958, Astrophys. J. Suppl., 3, 141 (1958ApJS....3..141B)

Balázs, J. 1935, Astron. Nachr., 254, 75 (1935AN....254...75B)

Balázs, J. 1936a, Astron. Nachr., 258, 305 (1936AN....258..305B)

Balázs, J. 1936b, Beobachtungs-Zirkular der AN, 18, (Nr.7), 14

Balázs, J. 1936c, Beobachtungs-Zirkular der AN, 19, (Nr.4), 7

Balázs, J. 1937a, Astron. Nachr., 261, 129 (1937AN....261..129B)

Balázs, J. 1937b, Astron. Nachr., 262, 437 (1937AN....262..437B)

Balázs, J. 1937c, Astron. Nachr., 262, 441 (1937AN....262..441B)

Balázs, J. 1938, Astron. Nachr., 265, 69 (1938AN....265...69B)

Balázs, J. 1952a, Mitt. Sternwarte Budapest, 30, 1 CoKon30

Balázs, J. 1952b, Mitt. Sternwarte Budapest, 30, 8 CoKon30

Balázs, J. 1955, Mitt. Sternwarte Budapest, 39, 6 CoKon39

Balázs, J. 1956, Mitt. Sternwarte Budapest, 40, 8 CoKon40

Balázs, J. 1957, Mitt. Sternwarte Budapest, 42, 99 CoKon42

Balázs, J. 1960, Kleine Veröff. Bamberg, Nr.27, 26

Balázs, J. 1963, Astron. Soc. of the Pacific Leaflet, No.417, 1

Balázs, J., Detre, L., 1934, Beobachtungs-Zirkular der AN, 16, (Nr.38) 72

Balázs, J., Detre, L., 1935, Astron. Nachr., 256, 87 (1935AN....256...87B)

Balázs, J., Detre, L., 1938, Mitt. Sternwarte Budapest, 5, 1 CoKon5

Balázs, J., Detre, L., 1939, Mitt. Sternwarte Budapest, 8, 1 CoKon8

Balázs, J., Detre, L., 1940, Mitt. Sternwarte Budapest, 11, 1 CoKon11

Balázs, J., Detre, L., 1941, Astron. Nachr., 271, 231 (1941AN....271..231B)

Balázs, J., Detre, L., 1943, Mitt. Sternwarte Budapest, 18, 1 CoKon18

Balázs, J., Detre, L., 1949, Mitt. Sternwarte Budapest, 21, 1 CoKon21

Balázs, J., Detre, L., 1950, Mitt. Sternwarte Budapest, 23, 1 CoKon23

Balázs, J., Detre, L., 1952, Mitt. Sternwarte Budapest, 27, 1 CoKon27

Balázs, J., Detre, L., 1954, Mitt. Sternwarte Budapest, 33, 1 CoKon33

Balázs, J., Detre, L., 1957, Mitt. Sternwarte Budapest, 34, 1 CoKon34

Balázs, J., Detre, L., 1961a, Mitt. Sternwarte Budapest, 50, 1 CoKon50

Balázs, J., Detre, L., 1961b, Acta Astronomica Sinica, 9, 77 (1961AcASn...9...77B)

Balázs, J., Detre, L., 1962, Kleine Veröff. Remeis-Sternwarte Bamberg, Nr.34, 90

Balázs, J., Detre, L., 1965, Kleine Veröff. Remeis-Sternwarte Bamberg, 4, (Nr.40) 184

Barlai, K., Szabados, L., Szeidl, B., 1972, Inf. Bull. Var. Stars, 640, 2 (IBVS N°.640)

Barlai, K., Szeidl, B., 1965, Mitt. Sternwarte Budapest, 56, 1 CoKon56

Barlai, K., 1989, Mitt. Sternwarte Budapest, 92, 143 CoKon92

Batten, A.H., Szeidl, B., 1972, Publ. Dominion Astrophys. Obs., 14, No.5, 97

Batyrev, A.A., 1957, Peremennye Zvezdy, 12, 137 (1957PZ.....12..137B)

Batyrev, A.A., 1964, Peremennye Zvezdy, 15, 278 (1964PZ.....15..278B)

Beyer, M., 1934, Astron. Nachr., 252, 85 (1934AN....252...85B)

Blazhko, S., 1907, Astron. Nachr., 175, 325 (1907AN....175..325B)

Blazhko, S., 1922, Annales de l'Observatorie de Moscou, 2-me serie, Vol. VIII. No. 2

Blazhko, S., 1922, Astron. Nachr., 216, 103 (1922AN....216..103B)

Blazhko, S., 1932, Leningrad Eph. of short period Ceph.

Bodokia, V.M., 1937, Abastumani Bull., 1

Bohlin, K., 1923, Astron. Nachr., 221, 195 (1924AN....221..195B)

Bohlin, K., 1925, Astron. Nachr., 224, 403 (1925AN....224..403B)

Bougoslawski, N., 1927, Astron. Nachr., 229, 203 (1927AN....229..203B)

Broglia, P., Masani, A., 1957, Contr. Oss. Astr. Milano-Merate, Nuova Serie, No. 105

Broglia, P., Pestarino, E., 1955, Mem. Soc. Astr. Ital., 26, 429 (1955MmSAI..26..429B)

Brosche, P., Zsoldos, E., 2003, Acta Historica Astronomiae, 18, 182 (2003AcHA...18..182B)

Chandler, S.C., 1896, Astron. J., 16, 145 (1896AJ.....16..145C)

Cocito, G., Masani, A., 1960, Contr. Oss. Astr. Torino, Nuova Serie, No. 27

Courvoisier, L., 1918, Astron. Nachr., 207, 17

Dahlgren, E., 1963, BAV Rundbrief, 12, 3 (1963BAVSR..12....3D)

Demers, S., Fernie, J.D., 1966, Astrophys. J., 144, 440 (1966ApJ...144..440D)

Detre, L., 1931, Beobachtungs-Zirkular der AN, 13, (Nr.24) 42

Detre, L., 1932a, Astron. Nachr., 246, 361 (1932AN....246..361D)

Detre, L., 1932b, Astron. Nachr., 246, 363 (1932AN....246..363D)

Detre, L., 1932c, Beobachtungs-Zirkular der AN, 14, (Nr.34) 61

Detre, L., 1933a, Astron. Nachr., 247, 309 (1933AN....247..309D)

Detre, L., 1933b, Astron. Nachr., 249, 213 (1933AN....249..213D)

Detre, L., 1934a, Astron. Nachr., 251, 27 (1934AN....251...27D)

Detre, L., 1934b, Beobachtungs-Zirkular der AN, 16, (Nr.18) 33

Detre, L., 1934c, Astron. Nachr., 252, 327 (1934AN....252..327D)

Detre, L., 1935a, Astron. Nachr., 254, 17 (1935AN....254...17D)

Detre, L., 1935b, Astron. Nachr., 254, 21 (1935AN....254...21D)

Detre, L., 1935c, Astron. Nachr., 257, 361 (1935AN....257..361D)

Detre, L., 1936a, Astron. Nachr., 258, 329 (1936AN....258..329D)

Detre, L., 1936b, Astron. Nachr., 259, 305 (1936AN....259..305D)

Detre, L., 1936c, Beobachtungs-Zirkular der AN, 18, (Nr.1) 2

Detre, L., 1936d, Beobachtungs-Zirkular der AN, 18, (Nr.4) 8

Detre, L., 1936e, Beobachtungs-Zirkular der AN, 18, (Nr.38) 67

Detre, L., 1937a, Astron. Nachr., 261, 9 (1937AN....261....9D)

Detre, L., 1937b, Astron. Nachr., 262, 81 (1937AN....262...81D)

Detre, L., 1940, Mitt. Sternwarte Budapest, 10, 1 CoKon10

Detre, L., 1941, Astron. Nachr., 271, 225 (1941AN....271..225D)

Detre, L., 1942, Astron. Nachr., 273, 253 (1943AN....273..253D)

Detre, L., 1943, Mitt. Sternwarte Budapest, 17, 1 CoKon17

Detre, L., 1954, Die Sterne, 30, Heft 11/12, 214

Detre, L., 1955, Trudy 4. Soveschanija po voprosam kozmogonii, Izd. Akad. Nauk SSSR, Moskva, 389

Detre, L., 1956a, Vistas in Astronomy, 2, 1156 (1956VA......2.1156D)

Detre, L., 1956b, Mitt. Sternwarte Budapest, 40, 1 CoKon40

Detre, L., 1957, Mitt. Sternwarte Budapest, 42, 103 CoKon42

Detre, L., 1960, Kleine Veröff Remeis-Sternwarte Bamberg, Nr.27, 23

Detre, L., 1962a, IAU Transactions, XI.B, 293

Detre, L., 1962b, Inf. Bull. Var. Stars, 13, 1 (IBVS N°.13)

Detre, L., 1965a, Sterne und Weltraum, 4, 157

Detre, L., 1965b, Astronomische Abhandlungen zum 70. Geburtstag von Prof. C. Hoffmeister, Johann Ambrosius Barth Verlag, Leipzig, 62

Detre, L., 1966a, Rev. Sc. Habana, 30

Detre, L., 1966b, Inf. Bull. Var. Stars, 152, 3 (IBVS N°.152)

Detre, L., 1967, Nordisk Astronomisk Tidskrift, 1, 7

Detre, L., 1969a, Inf. Bull. Var. Stars, 335, 2 (IBVS N°.335)

Detre, L., 1969b, Inf. Bull. Var. Stars, 380, 1 (IBVS N°.380)

Detre, L., 1969c, in Non-Periodic Phenomena in Variable Stars, IAU Coll. No.4, (ed. L.Detre) Akadémiai Kiadó, 3 CoKon65

Detre, L., 1970a, IAU Transactions, XIV.A, 259

Detre, L., 1970b, Ann. Univ.-Sternwarte Wien 29, (Nr.2) 79

Detre, L., 1973, IAU Transactions, XV.A, 333

Detre, L., 1974, Astrophys. and Space Sci. Library, 45, 51 (1974ssr..conf...51D)

Detre, L., Chang, Y.C., 1960, Acta Astronomica Sinica, 8, 50 (1960AcASn...8...50D)

Detre, L., Herczeg, T., 1952, Mitt. Sternwarte Budapest, 29, 1 CoKon29

Detre, L., Kanyó, S., 1961, Mitt. Sternwarte Budapest, 49, 1 CoKon49

Detre, L., Lassovszky, K., 1934a, Astron. Nachr., 252, 197 (1934AN....252..197D)

Detre, L., Lassovszky, K., 1934b, Astron. Nachr., 252, 201 (1934AN....252..201D)

Detre, L., Lassovszky, K., 1939, Mitt. Sternwarte Budapest, 9, 1 CoKon9

Detre, L., Szeidl, B., 1967, Inf. Bull. Var. Stars, 204, 3 (IBVS N°.204)

Detre, L., Szeidl, B., 1973a, Inf. Bull. Var. Stars, 764, 1 (IBVS N°.764)

Detre, L., Szeidl, B., 1973b, Astrophys. and Space Sci. Library, 36, 31 (1973vsgc.coll...31D)

Dezsö, L., 1945, Mitt. Sternwarte Budapest, 20, 1 CoKon20

Dezsö, L., 1949, Peremennye Zvezdy, 7, 30

Eddington, A.S., 1918, Mon. Not. RAS, 79, 2 (1918MNRAS..79Q...2E)

Eddington, A.S., 1919, Mon. Not. RAS, 79, 177 (1919MNRAS..79R.177E)

Fath, E.A., 1935, Lick Obs. Bull., No. 479

Fath, E.A., 1937, Lick Obs. Bull., No. 487

Fernie, J.D., Hube, J.C., 1971, Astrophys. J., 168, 437 (1971ApJ...168..437F)

Fitch, W.S., Szeidl, B., 1976, Astrophys. Journal, 203, 616 (1976ApJ...203..616F)

Fitch, W.S., Wisniewski, W.Z., Johnson, H.L., 1966, Commun. Lunar and Planetary Lab., No. 71, Vol. 5, Part 2

Florja, N.F., 1934, Leningrad Bull., 4

Florja, N.T., 1937, Astron. Zhurnal, 14, 1

Gefferth, K., Szeidl, B., 1962, Inf. Bull. Var. Stars, 7, 1 (IBVS N°.7)

Geyer, E.H., Szeidl, B., 1965, Kleine Veröff. Remeis-Sternw. Bamberg, 4, (Nr.40) 63

Geyer, E.H., Szeidl, B., 1970, Astron. Astrophys., 4, 40 (1970A&A.....4...40G)

Geyer, E., Kippenhahn, R., Strohmeier, W., 1955, Kleine Veröff. Bamberg, Nr. 11

Gothard, J., 1887, Zeitschrift fur Instrumentenkunde, S. 347

Guman, I., 1951, Mitt. Sternwarte Budapest, 24, 1 CoKon24

Guman, I., 1952, Mitt. Sternwarte Budapest, 31, 1 CoKon31

Guman, I., 1955a, Mitt. Sternwarte Budapest, 36, 1 CoKon36

Guman, I., 1955b, Mitt. Sternwarte Budapest, 39, 10 CoKon39

Guman, I., 1982, Mitt. Sternwarte Budapest, 78, 1 CoKon78

Guthnick, P., Prager, R., 1927, Astron. Nachr., 229, 455 (1927AN....229..455G)

Hagen, J.G., 1905, Astron. Nachr., 168, 11 (1905AN....168Q..11H)

Hardie, R.H., 1955, Astrophys. J., 122, 256 (1955ApJ...122..256H)

Harkányi, B., 1901a, Astron. Nachr., 155, 155 (1901AN....155..155H)

Harkányi, B., 1901b, Astron. Nachr., 156, 79 (1901AN....156...79H)

Harkányi, B., 1903, Mathematische und Naturwissenschaftliche Berichte aus Ungarn, 19, 31

Harkányi, B., 1910, Astron. Nachr., 186, 161 (1910AN....186..161H)

Hartwig, E., 1908, Astron. Nachr., 177, 70 (1908AN....177...69H)

Herczeg, T., 1956a, Mitt. Sternwarte Budapest, 41, 18 CoKon41

Herczeg, T., 1956b, Mitt. Sternwarte Budapest, 41, 23 CoKon41

Herczeg, T., 1957a, Mitt. Sternwarte Budapest, 35, 39 CoKon35

Herczeg, T., 1957b, Mitt. Sternwarte Budapest, 42, 39 CoKon42

Hertzsprung, E., 1922, Bull. Astron. Netherl., 1, 139 (1922BAN.....1..139H)

Hoffmeister, C., 1949, Astron. Abh., 12, 21 (1949AAHam..12...21H)

Illés, E., 1960, Astr. Tsirk., 210, 21 (1960ATsir.210...21I)

Illés, E., 1963, Inf. Bull. Var. Stars, 22, 1 (IBVS N°.22)

Illés, E., 1968, Inf. Bull. Var. Stars, 303, 1 (IBVS N°.303)

Illés, E., Almár, I., 1963a, Acta Astronomica, 13, 72 (1963AcA....13...72I)

Illés, E., Almár, I., 1963b, Acta Astronomica, 13, 75 (1963AcA....13...75I)

Illés, E., Szabados, L., 1976, in Multiple Periodic Variable Stars, Proc. IAU Coll. No.29, Contributed Papers (Ed. W.S. Fitch) 165

Ivanov, N., 1924, Astron. Nachr., 223, 287 (1925AN....223..287I)

Ivanov, N., 1926, Astron. Nachr., 228, 143 (1926AN....228..143I)

Izsák, I., 1952, Mitt. Sternwarte Budapest, 28, 1 CoKon28

Izsák, I., 1957, Mitt. Sternwarte Budapest, 42, 63 CoKon42

Jankovics, I., 1973, Inf. Bull. Var. Stars, 840, 1 (IBVS N°.840)

Jordan, F.C., 1929, Allegh. Publ., 7, 1

Jordan, F.C., 1929, Allegh. Publ., 7, 34

Kanyó, S., 1966, Inf. Bull. Var. Stars, 146, 1 (IBVS N°.146)

Kanyó, S., 1970, Inf. Bull. Var. Stars, 490, 1 (IBVS N°.490)

Kanyó, S., 1972, Inf. Bull. Var. Stars, 737, 1 (IBVS N°.737)

Kanyó, S., 1976a, Mitt. Sternwarte Budapest, 69, 1 CoKon69

Kanyó, S., 1976b, in Multiple Periodic Variable Stars, Proc. IAU Coll. No.29, Contributed Papers (Ed. W.S. Fitch) 211

Kanyó, S., Szeidl, B. 1974, Mitt. Sternwarte Budapest, 64, 1 CoKon64

Kato, S., 1958, Tokyo Astronomical Bull., Ser. II, No. 110

Kwee, K.K., 1958, Transactions of IAU, vol. 10, p. 638

Lange, A.G., 1935, Tadjik Tsirk., 4

Lange, A.G., 1943, Astron. Tsirk., No. 20

Lassovszky, K., 1931a, Beobachtungs-Zirkular der AN, 13, (Nr. 30) 51

Lassovszky, K., 1931b, Astron. Nachr., 243, 399 (1931AN....243..399L)

Lassovszky, K., 1931c, Astron. Nachr., 243, 404 (1931AN....243..399L)

Lassovszky, K., 1932a, Astron. Nachr., 245, 349 (1932AN....245..349L)

Lassovszky, K., 1932b, Astron. Nachr., 246, 289 (1932AN....246..289L)

Lassovszky, K., 1933a, Astron. Nachr., 247, 177 (1933AN....247..177L)

Lassovszky, K., 1933b, Astron. Nachr., 247, 182 (1933AN....247..177L)

Lassovszky, K., 1933c, Astron. Nachr., 248, 313 (1933AN....248..313L)

Lassovszky, K., 1934, Astron. Nachr., 252, 221 (1934AN....252..221L)

Lassovszky, K., 1935a, Astron. Nachr., 254, 25 (1935AN....254...25L)

Lassovszky, K., 1935b, Astron. Nachr., 256, 167 (1935AN....256..167L)

Lassovszky, K., 1936a, Astron. Nachr., 258, 93 (1936AN....258...93L)

Lassovszky, K., 1936b, Astron. Nachr., 259, 343

Lassovszky, K., 1936c, Astron. Nachr., 260, 94

Lassovszky, K., 1938, Mitt. Sternwarte Budapest, 6, 1 CoKon6

Lause, F., 1932, Astron. Nachr., 245, 329 (1932AN....245..329L)

Lause, F., 1933, Astron. Nachr., 249, 377 (1933AN....249..377L)

Lovas, M., 1952, Mitt. Sternwarte Budapest, 31, 4 CoKon31

Lovas, M., 1965, Inf. Bull. Var. Stars, 99, 1 (IBVS N°.99)

Mahdy, H.A., Szeidl, B., 1980, Mitt. Sternwarte Budapest, 74, 1 CoKon74

Martin, C., Plummer, H.C., 1921, Mon. Not. RAS, 81, 458 (1921MNRAS..81..458M)

Martin, W.Ch., 1938, Leiden Ann., 17, Part 2

Martynov, D.J., 1940, Engelhardt Obs. Bull., 18

Móra, K., 1930, Math. Nat. Anzeiger d. Ung. Akad. d. Wiss., 47, 403

Móra, K., 1934, Mitt. Sternwarte Budapest, 3, 1 CoKon3

Münch, G., 1951, Astrophys. J., 114, 546 (1951ApJ...114..546M)

Nakamura, K., 1922, Kyoto Bull., 8, 10

Nielsen, A.V., 1936, IAU Circ., No. 594

Oláh, K., 1974, Inf. Bull. Var. Stars, 987, 1 (IBVS N°.987)

Oláh, K., Szeidl, B., 1978, Mitt. Sternwarte Budapest, 71, 1 CoKon71

Onderlicka, B., Vetesnik, M., 1968, Publ. Astron. Inst. Univ. Brno, No. 8

Oosterhoff, P.Th., 1930, Bull. Astron. Inst. Netherl., 6. 39 (1930BAN.....6R..39O)

Ozsváh, I., 1957, Mitt. Sternwarte Budapest, 42, 81 CoKon42

Patkós, L., 1975, Inf. Bull. Var. Stars, 1065, 1 (IBVS N°.1065)

Patkós, L., 1976, Inf. Bull. Var. Stars, 1200, 1 (IBVS N°.1200)

Payne-Gaposchkin, C., 1952, Harvard Ann., 118, No. 15

Pickering, E.C., 1901, Astron. Nachr., 154, 405 (1901AN....154..405P)

Pickering, E.C., 1901, Harvard Circ., No. 54

Pigott, E., 1797, Philosophical Transactions of the Royal Society, 133

Pocs, M., Szeidl, B., 2001, Astron. Astrophys., 368, 880 (2001A&A...368..880P)

Pocs, M., Szeidl, B., Viraghalmy, G., 2002, Astron. Astrophys., 393, 555 (2002A&A...393..555P)

Prager, R., 1936, Geschichte und Literatur des Lichtwechsels der Veranderlichen Sterne, 1, 109

Preston, G.W., Smak, J., Paczynski, B., 1965, Astrophys. J. Suppl., 12, 99 (1965ApJS...12...99P)

Robinson, L.J., 1966, Peremennye Zvezdy, 16, 62 (1966PZ.....16...62R)

Robinson, L.V., 1930, Harvard Bull., 876

Robinson, L.V., 1930, Harvard Bull. No. 867

Robinson, L.V., 1930, Harvard Bull., No. 872

Schneller, H., 1928, Astron. Nachr., 233, 41

Schneller, H., 1961, Astron. Nachr., 286, 102 (1961AN....286..102S)

Schwab, F., 1918, Astron. Nachr., 207, 17

Shapley, H., 1914, Astrophys. J., 40, 448 (1914ApJ....40..448S)

Shapley, H., 1916, Astrophys. J., 43, 217 (1916ApJ....43..217S)

Shapley, H., 1921, Mon. Not. RAS, 81, 208 (1921MNRAS..81..208S)

de Sitter, A., 1932, Bull. Astron. Inst. Netherl., 6, 215 (1932BAN.....6..215D)

Sterne, T.E., 1934, Harvard Circ., No. 387

Strohmeier, W., 1963, Inf. Bull. Var. Stars, 26, 1. (IBVS N°.26)

Strohmeier, W., Knigge, R., 1960, Veröffentl. Bamberg, 5, Nr. 6

Szabados, L., 1974a, Inf. Bull. Var. Stars, 867, 1 (IBVS N°.867)

Szabados, L., 1974b, Inf. Bull. Var. Stars, 868, 1 (IBVS N°.868)

Szabados, L., 1976a, In Multiple Periodic Variable Stars, Proc. IAU Coll. No.29, Contributed Papers (Ed. W.S. Fitch) 159

Szabados, L., 1976b, Inf. Bull. Var. Stars, 1107, 1 (IBVS N°.1107)

Szabados, L., Stobie, R.S., Pickup, D.A., 1976, Inf. Bull. Var. Stars, 1197, 1 (IBVS N°.1197)

Szeidl, B., 1963, Inf. Bull. Var. Stars, 36, 1 (IBVS N°.36)

Szeidl, B., 1965, Mitt. Sternwarte Budapest, 58, 1 CoKon58

Szeidl, B., 1967, Inf. Bull. Var. Stars, 220, 1 (IBVS N°.220)

Szeidl, B., 1968a, Inf. Bull. Var. Stars, 278, 1 (IBVS N°.278)

Szeidl, B., 1968b, Inf. Bull. Var. Stars, 291, 1 (IBVS N°.291)

Szeidl, B., 1969a, Inf. Bull. Var. Stars, 345, 1 (IBVS N°.345)

Szeidl, B., 1969b, Inf. Bull. Var. Stars, 385, 1 (IBVS N°.385)

Szeidl, B., 1973, Mitt. Sternwarte Budapest, 63, 1 CoKon63

Szeidl, B., 1974, Inf. Bull. Var. Stars, 903, 1 (IBVS N°.903)

Szeidl, B., 1975, IAU Symp. No.67, 545 (1975IAUS...67..545S)

Szeidl, B., 1976, Astrophysics and Space Science Library, 60, 133 (1976mpvs.coll..133S)

Szeidl, B., 1983, Mitt. Sternwarte Budapest, 84, 251 CoKon84

Szeidl, B., Guinan, E.F., Oláh, K., Szabados, L., 1997, Mitt. Sternwarte Budapest, 99, 1 CoKon99

Szeidl, B., Jurcsik, J., Benko, J.M., Bakos, G., 2001, Mitt. Sternwarte Budapest, 101, 1 CoKon101

Szeidl, B., Mahdy, H.A., 1981, Mitt. Sternwarte Budapest, 75, 1 CoKon75

Szeidl, B., Oláh, K., Barlai, K., Szabados, L., 2001, Mitt. Sternwarte Budapest, 102, 1 CoKon102

Szeidl, B., Oláh, K., Mizser, A., 1986, Mitt. Sternwarte Budapest, 89, 57 CoKon89

Szeidl, B., Oláh, K., Szabados, L., Barlai, K., Patkós, L., 1992, Mitt. Sternwarte Budapest, 97, 245 CoKon97

Szeidl, B., Viraghalmy, G., 2000, Mitt. Sternwarte Budapest, 98, 303 CoKon98

Tass, A., 1904a, Astron. Nachr., 165, 177 (1904AN....165..177T)

Tass, A., 1904b, Kleinere Veröffentlichungen des O-Gyallaer Astrophysikalischen Observatoriums Stiftung v. Konkoly, Nr. 3, 1

Tass, A., 1905a, Astron. Nachr., 167, 109 (1905AN....167..109T)

Tass, A., 1905b, Astron. Nachr., 168, 197 (1905AN....168..197T)

Tass, A., 1905c, Astron. Nachr., 168, 207 (1905AN....168..207T)

Tass, A., 1905d, Astron. Nachr., 168, 321 (1905AN....168..321T)

Tass, A., 1906, Astron. Nachr., 173, 145 (1907AN....173..145T)

Tass, A., 1908a, Astron. Nachr., 179, 33 (1909AN....179...33T)

Tass, A., 1908b, Astron. Nachr., 179, 341 (1909AN....179..341T)

Tass, A., 1918, Astron. Nachr., 207, 65

Tass, A., 1925, Publ. Astrophysikalischen Observatoriums Stiftung v. Konkoly, Budapest, 2, (Nr.2) 1

Tass, A., Terkán, L., 1916, Publ. des O-Gyallaer Astrophysikalischen Observatoriums Stiftung v. Konkoly, 1, (Nr.1) 1

Tchumak, O.V., 1965, Peremennye Zvezdy, 15, 569 (1965PZ.....15..569T)

Terkán, L., 1904, Kleinere Veröffentlichungen des O-Gyallaer Astrophysikalischen Observatoriums Stiftung v. Konkoly, Nr.6, 1

Terkán, L., 1905, Astron. Nachr., 168, 33 (1905AN....168...33T)

Terkán, L., 1906a, Astron. Nachr., 170, 171 (1906AN....170..171T)

Terkán, L., 1906b, Kleinere Veröffentlichungen des O-Gyallaer Astrophysikalischen Observatoriums Stiftung v. Konkoly, Nr.10, 1

Terkán, L., 1910, Astron. Nachr., 186, 113 (1911AN....186..113T)

Terkán, L., 1926, Astron. Nachr., 226, 345 (1926AN....226..345T)

Tremko, J., 1964, Mitt. Sternwarte Budapest, 55, 1 CoKon55

Tremko, J., 1974, Mitt. Sternwarte Budapest, 67, 1 CoKon67

Tsessevich, V.P., 1953, Odessa Isv., 3, 257

Tsessevich, V.P., 1973, Astron. Tsirk., 775

Tsessevich, V.P., 1966, Astron. Tsirk., No. 353, 3

Underhill, A.B., 1966, The Early Type Stars, D. Reidel Publ. Co. Dordrecht, p. 259

Walraven, Th., 1949, Bull. Astron. Netherl., 11, 17

Williams, A.S., 1926, Mon. Not. RAS, 87, 172 (1926MNRAS..87..172W)

Wozniak, P.R., Vestrand, W.T., Akerlof, C.W., et al., 2004, Astron. J., 127, 2436 (2004AJ....127.2436W)

Zinner, E., 1912, Astron. Nachr., 190, 377 (1912AN....190..377Z)

Zinner, E., 1916, Astron. Nachr., 202, 233 (1916AN....202..233Z)