Non-Periodic Phenomena in Variable Stars
                                                IAU Colloquium, Budapest, 1968

       PHOTOELECTRIC OBSERVATIONS OF 6 SOUTHERN RW AURIGAE
                            VARIABLES

                 W. SEGGEWISS and E. H. GEYER
                        Sternwarte Bonn

  A program for investigating the photometric behaviour of RW Aurigae 
variables has been started in 1962 by one of the authors (E.H.G.) at the 
Boyden Observatory in South Africa. The observations in the UBV colour 
system were carried out with the photoelectric equipment of the 60 inch 
Rockefeller reflector.

  The following six stars classified as RW Aurigae type variables in the 
GCVS (1958) have been selected:
			
            Star              l^II     (1950)   b^II

            SY  Phe          278.5 deg         -72.5 deg
            SZ  Phe          277.5             -71.7    
            TT  Phe          269.7             -71.5
            BS  Vel          260.0             - 1.5
            DI  Car          294.9             - 8.6
            ES  Car          290.7             + 0.2
		
  On the average these stars were measured three times per night during a 
total of 8 to 17 nights from 1962 to 1964.
  The first four stars show only small changes in magnitude (Fig. 1) and 
colours confirming the observations by Hoffmeister (1958) and De Kort 
(1941). In Figure 2 the position of these stars in the two colour 
diagram is plotted. It seems that SY Phe is an F-Type star in agreement 
with the spectral type given in the HD-catalogue (F8) and the Potsdamer 
Spektraldurchmusterung (F4). For SZ Phe the PSD gives the spectral type 
K4. For the other two stars no spectral types are available. From their 
position in the diagram it is concluded that they are also K-type stars.
  The star DI Car shows a brightness variation larger than 0.7m in V and 
0.3m in B - V, resp. 0.34m in U - B (Fig. 3). The light curve resembles 
that of a cepheid star with a period of approximately 30 days. Yet Hoffmeister 
(1957) considered this star a member of the CN Ori variables, which are 
a subgroup of the U Gem stars. In the Henry Draper Extension the spectrum 
of DI Car is characterized as peculiar. We found a weak objective prism 
spectrum of this variable on a plate taken by one of us with the duplicate 
of the original Schmidt camera at the Boyden Observatory. Because of 
the faintness it is almost impossible to associate a correct spectral type. 
Three features, however, seem important assuming that they are not simulated 
by spots on the plate:

  

                             Fig. 1

  

                             Fig. 2


  (1) Sr II 4125 seems absent, as concluded from the undiminished strength 
of the continuum in this region.
  (2) A strong absorption feature merges into the G-band from the red side; 
it may be interpreted as H gamma which is, however, too strong for the 
late type apparent from the H and K lines and Ca I.
  (3) The continuum between H beta and H gamma is extremely weak. Tentatively 
we might assume the star to be a K-dwarf, possibly composite.
  During its variations the star moves along a straight line above the 
main sequence in the two colour diagram, in the colour magnitude 
diagrams the star describes loops (Fig. 4).
  It is known through the photographic observations of Hertzsprung 
(1925) that the star ES Car shows besides isolated brightness variations 
up to 2 magnitudes small and rapid variations of 0.5m, which are 
confirmed by our measurements (Fig. 5). In Figure 5 the individual 
observations of each night are plotted. The magnitude and colour 
differences in the sense variable minus comparison star are given. The 
relevant data for the comparison star are: V = 12.78m, B - V = 0.28m, 
U - B = 0.14m. This variable is situated close to the galactic plane in 
a very rich and complicated portion of the southern Milky Way studied by 
Schmidt and Diaz Santanilla (1964). They found in the NW edge of the 
open cluster NGC 3572 a second very young cluster located at a larger 
distance than NGC 3572 which they designated NGC 3572 b. ES Car lies 
close to the centre of this second cluster (star No. 33 in the paper of 
Schmidt and Diaz Santanilla). Figure 6 is taken from the above publication. 
The variable is shown as a square, filled circles are cluster members, open 
circles probable members, and crosses field stars. Assuming ES Car a cluster 
member, the following absolute magnitude and intrinsic colours can be derived

  
 
                             Fig. 3

  

                             Fig. 4


from the known data of NGC 3572b: M_v = -1.65m, (B -V)_0 = -0.14m, 
(U - B)_0 = -0.32m. This corresponds to a B7III star.
  The position of the variable in the colour-magnitude diagrams is above 
the main sequence indicating the possibility that the star is still in 
the contracting phase of its evolutionary track.
  In this case NGC 3572b would be the cluster with the earliest not yet to 
the main sequence contracted member.

  

                             Fig. 5  

  

                             Fig. 6


  We are indebted to W. S. Pretorius, former night assistant at Boyden 
Observatory, for carrying out most of the photoelectric observations.


                             REFERENCES

Hertzsprung, E., 1925, Bull. astr. Inst. Netherl. 210.
Hoffmeister, C., 1957, private communication, in Schneller, H., Geschichte und 
             Literatur des Lichtwechsels der Veränderlichen Sterne, Vol. 4, Berlin. 
Hoffmeister, C., 1958, Veröff. Sonneberg 3, 348.
De Kort, J., 1941, Bull. astr. Inst. Netherl. 9, 245.
Kukarkin, B. V. et al., 1958, General Catalogue of Variable Stars, Moscow. 
Schmidt, H. and G. Diaz Santanilla, 1964, Veröff. Bonn No. 71.