COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                       Number 3305

                                          Konkoly Observatory
                                          Budapest
                                          15 March 1989

                                          HU ISSN 0374 - 0676


                 Light Curves for CG Cygni


Since the discovery of its variability by Stanley Williams (1922), CG Cygni has
been the object of intense study. More recently Milone has shown that the
system clearly has a migrating maculation ("Starspot") wave in its light curve
(see Milone et al., 1979). This bulletin details a re-analysis of Jassurs
(1980) 1979 BVR data, employing the method and computer programs outlined by
Budding and Zeilik (1987) who analysed a 1981 V Band light curve of this star.
Jassur only studied the eclipses in his study.

The starting parameters were taken from Budding and Zeilik, in the expectation
that once the maculation wave has been accounted for, the geometric parameters
will remain constant with time. Initial fits to the distorted light curves
produced the following parameters:

Primary Radius r_1    =    0.214
Secondary Radius r_2  =    0.224
Primary Luminosity L_1=   60.08 %

We fixed the inclination at the final Budding and Zeilik value of 82.8 degrees
for this initial model.

We then examined the difference curve obtained by subtracting this initial
model from the light curve, and attempted to model it as a maculation wave.
Two middle latitude spot groups of similar sizes (about 18 degrees in radius)
located around the longitudes 126deg and 246deg were found to fit the data best.
Following the procedure of Budding and Zeilik the spot fluxes were set to zero,
thus producing minimum area spots. When these results are compared with those
of Budding and Zeilik it appears that while the first spot had remained static
between 1979 and 1981, the second increased in both radius and longitude.

The final light curves were formed by subtracting the calculated maculation
effects from the original data. The basic parameters specifying this fit are:

Primary Radius r_1     = 0.223
Secondary Radius r_2   = 0.238
Inclination i          = 84deg
Primary Luminosity L_1 = 61.9%

The correlated errors calculated for these quantities are of order 1%. At
first glance these appear to be appreciably different from those found by
Budding and Zeilik:

Primary Radius r_1     = 0.241
Secondary Radius r_2   = 0.226
Inclination i          = 83deg
Primary Luminosity L_1 = 74.5%


                            [FIGURE 1]

 Figure 1: The original V Band data is plotted against the best initial
           model fit (the straight line).

                            [FIGURE 2]

 Figure 2: The residuals from the initial V Band fit are plotted with the
           model light curve produced by the two dark spots discussed in
           the text.

                            [FIGURE 3]

 Figure 3: The final model light curve is plotted against the spot
           corrected V Band data.


Mancuso et al. (1981) noted a genuine ambiguity between the transit and
occultation hypothesis for systems with a mass ratio in the range between 0.79
and unity, as CG Cyg's does. They also showed that while all transit
hypothesis minima could be at least roughly simulated by an occultation, the
reverse is not true. In these ambiguous cases while the inclination of the
system remains constant, the stellar radii are essentially transposed and the
occultation Primary luminosity is that of the Transit case multiplied by the
square of the ratio of the radii (r_2/r_1). Thus the present study's results
would be:

Primary Radius r_1     =  0.238
Secondary Radius r_2   =  0.223
Inclination i          =  83.0deg
Primary Luminosity L_1 =  70.5 %

These parameters were found to well within error of the results of Budding and
Zeilik, and in view of the spectroscopically supported discussion of the Main
Sequence-like character of the stars given by these authors, the transit
solution would clearly give the preferable physical interpretation.

The results of this study can be seen to be in agreement with the transit model
of Budding and Zeilik, and lends further support to their modeling procedure
for RS CVn systems. However, the photometric evidence alone does show that the
alternative occultation model (as considered by Jassur) is also feasible. The
authors acknowledge Professor Zeilik's correspondence on this subject.


TIMOTHY BANKS        and EDWIN BUDDING
Victoria University      Carter Observatory
of Wellington            Wellington
New Zealand              New Zealand

References:

Budding E., and Zeilik M., Astrophys J., 319, 827 - 835,1987. [BIBCODE 1987ApJ...319..827B ]
Jassur, D.M.Z., Astrophys Spa. Sci., 67, 31 - 43, 1980. [BIBCODE 1980Ap&SS..67...19J ]
Mancuso, S. Milano, L., Vottore, A., Budding E. Jassur D.M.Z. in Photometric
   and Spectroscopic Binary Systems, E.B. Carling and Z. Kopal (Eds), D.
   Reidel Publishing Co., 1981 [BIBCODE 1981psbs.conf..313M ]
Milone, E.F., Castle, C.G., Robb, R.M., Swadron, D., Burke, E.W., Hall, D.S.,
   Michlovic, J.E., and Zissel, R.E., Astron. J., 84(1), 417, 1979. [BIBCODE 1979AJ.....84..417M ]
Williams, A.S., Mon. Not. Roy. Astr. Soc., 82, 300, 1922. [BIBCODE 1922MNRAS..82..300W ]