Binary stars among Galactic classical Cepheids


This site contains useful information on those Cepheids in the Milky Way galaxy which are known to have a physical companion (or more companions). When compiling this list, the published literature was critically reviewed, and whenever new pieces of information are available, the site is revised, updated and extended.

The announcement of this database appeared in the Information Bulletin on Variable Stars. When referring to this site in published papers please include the following citation: Szabados L., 2003, Inf. Bull. Var. Stars, No. 5394.

Other important data of Galactic Cepheids (not only on binaries) are available in the Database of Galactic Classical Cepheids maintained by the David Dunlap Observatory. Click here ( to consult the DDO Database.

Click here ( for retrieving information from McMaster Cepheid Photometry and Radial Velocity Data Archive.

The site is intended to give easily accessible background information for those who are involved in performing or analysing photometric or spectroscopic observations of classical (i.e. Pop. I) Cepheids.

As far as photometric data are concerned, the effect of the companion is essential in deriving the apparent brightness and the intrinsic colour indices of the Cepheid. If the effect of a blue main-sequence companion is not taken into account (i.e. the observed brightness is attributed solely to the Cepheid), the apparent magnitude and colour indices can be falsified by several hundredth of a magnitude. The false (bluer) colour introduces an error in the derivation of the interstellar extinction, mimicking a smaller amount of absorption. Together with the brighter apparent magnitude (also due to the companion) the Cepheid seems to be more luminous than it is in reality. In brief, negligence of the companion(s) leads to an erroneous zero-point of the period-luminosity relationship.

It would be an easy but unreasonable solution to exclude Cepheids belonging to binary systems from the calibration of the period-luminosity relation because majority of classical Cepheids have one or more companions (Szabados L., 2003, in GAIA Spectroscopy, Science and Technology, ed. U. Munari, ASP Conf. Ser., Vol. 298, 237-244).

Description of the tables

Main table

The individual columns contain:
1. Name of the Cepheid. This GCVS identification is linked to the SIMBAD Database (CDS, Strasbourg, France).
2. Decimal logarithm of the pulsation period.
3. Mean brightness in the V band of the Johnson UBV photometric system.
4. Spectral type of the companion.
5. Duplicity status. The meaning of the letters is as follows:
  B - spectroscopic binary (a colon denotes if confirmation is needed),
  b - photometric companion, physical relation should be investigated,
  O - spectroscopic binary with known orbit (linked to the table of the orbital elements),
  V - visual binary.
6. Occasional remark on the importance or peculiarity of the given Cepheid.
7. Comprehensive but not exhaustive list of references linked to the bibliographical part of the site. The system of coding of the references is as follows:
  - in the case of a single author, the first two letters of the author's name is followed by the last two digits of the year of publication;
  - in the case of two or more authors, the initial of each author's name is followed by the last two digits of the year of publication;

Table containing the orbital elements for binary Cepheids

One set of orbital elements starts with the name of the Cepheid and ends with the coded reference. The individual columns contain:
1. Name of the Cepheid.
2. Gamma-velocity (i.e. the mean velocity averaged for pulsation) in km/s.
3. Semi-amplitude of the projected orbital radial velocity variation, K, in km/s.
4. Eccentricity of the orbit, e.
5. Longitude of the periastron, omega.
6. Epoch of a periastron passage, T_0.
7. Orbital period, P_orb.
8. Mass function, f(M).
9. Projected semimajor axis, a sin_i, and in some cases the inclination, i, and semi-major axis, a.
10. Reference (linked to the bibliography).
If no reference is given, the set continues in the subsequent line where the errors of the corresponding quantities are given. The reference in this second line completes the set of elements.


The list of references first gives the complete bibliography arranged alphabetically following the code used in the main table and the table of the orbital elements. Papers referred to in this list that were published in major astronomy journals are directly linked to the CDS bibliographic service or ADS.


The content of this site was compiled in the frame of the OTKA project T029013. The HTML version was prepared by András Holl. Comments and additional information on new or already known binary Cepheids are welcome, in order to update the content of the tables to the address:


László Szabados

Latest modification: 17 June 2019