COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1662

                                      Konkoly Observatory
                                      Budapest
                                      1979 August 27




          STROMGREN y and b LIGHT CURVES OF W UMa





    The short period eclipsing system W UMa was observed on 10 nights from
April through July 1977. The observations were obtained using the 51 cm, f/13.5
Cassegrain reflector at Biruni Observatory, Shiraz, Iran. The photoelectric photometer 
is equipped with an unrefrigerated RCA 4509 multiplier photocell and a Leeds
and Northrup Speedomax was used to record the amplified signal from the photomultiplier. 
Intermediate-bandpass blue and yellow interference filters having
characteristics nearly identical with the b and y filters of the Stromgren uvby system
were employed in making the observations. The comparison star was BD+56d1399 and
BD+56d1398 served as the check star. The comparison star was the same used in
previous investigations of W UMa. At the level of +-0.004 mag in yellow and +-0.007,
mag in blue no significant variations were detected between the comparison and check
stars. Several uvby standard stars were also observed.

    The observing sequence was the usual pattern of sky-comparison-variable-
comparison-sky, with each observation consisting of a 40 sec deflection. The faint ~12.5
mag companion star ADS 7494 B, about 7" from W UMa, was included in all measurements 
of the variable. The effects of differential atmospheric extinction were removed,
but because of the angular proximity of the comparison star to the variable star, the
extinction corrections were always very small. The differential y and b magnitudes
in the sense (V-C) are plotted against phase in Figure 1 where the phases were
computed according to:   Min I = JD Hel. 2443863.7583 + 0.33363793* E.
The epoch is from the present study and the period was obtained from Rigterink (1972).

    A determination of the time of secondary minimum was made by combining
observations of four nights on which secondary eclipse was observed. The method of
Szafraniec (1948) was used to compute the timing, yielding the epoch:

JD Hel. Min II = 2442863.5915. No precise determination of the time of primary
eclipse could be made since only a portion of the ingress was observed. In computing
the phases it was assumed that primary and secondary minima are separated by
exactly 1/2 period.     

    The present light curves are well defined except for the descending branch of
primary minimum which is only partially covered. As shown in Figure 1, both the y

                                 [FIGURE 1]

            Figure 1.   The y and b light curves of W UMa.

and b light curves are asymmetrical where the maximum near 0.25 phase is brighter
than the corresponding maximum at 0.75 phase. The difference in the mean heights
of the maxima at 0.25 phase relative to the maxima at 0.75 phase are 0.022 mag and
0.012 mag for the y and b observations, respectively. The loss of light at primary
eclipse relative to the brighter maximum at 0.25 phase is 0.715 mag and 0.755 mag in
y and b, respectively. The loss of light at secondary minimum is 0.632 mag and 0.660
mag in y and b, respectively.

    Rigterink (1972) has made a study of the long term behavior of the outside
eclipse light variations of W UMa and found evidence that the changes may be periodic
with possible periods of ~500 days or ~1000 days. An analysis of all the available
photoelectric light curves of W UMa up to 1979 is currently underway. Preliminary
results suggest the existence of a migrating wave with a light amplitude of about 0.025
mag in yellow and a period of ~1100 days. It has already been pointed out by Hall (1976)
that W UMa has several properties in common with RS CVn-type binaries-namely
components of F to K spectral types, Ca II emission, and the possible migrating wave
phenomenon. It has been suggested by Hall and others that many of the unusual
properties of RS CVn systems can be accounted for by recourse to a model in which
surface activity in the form of starspots cooler than their surroundings gives rise to the
observed light variations. If the wave-like distortion apparently present in the light
variation of W UMa arises from subluminous regions on one of the components, the
migrating of the wave can be attributed to the effects of differential rotation where the
subluminous region is centered perhaps 10d to 20d from the stellar equator. By analogy
with the sun, differential rotation would cause the spotted region to move with a 
rotational period slightly longer than the equatorial rotational period. For W UMa where
the rotational and orbital periods are expected to be synchronized, the small difference
between orbital period and the rotational period of the spotted zone will cause the spotted
region to move with the observed ~1100 day period relative to the frame of reference
of the binary system. The observations indicate that the spotted region has persisted
for at least 25 years. Continued monitoring of the light curve of W UMa would be very
useful.



	EDWARD F. GUINAN                SEYED - IRAJ NAJAFI
	Biruni Observatory              and
	Shiraz, Iran. and               FARID ZAMANI - NOOR
	Department of Astronomy         Biruni Observatory
	Villanova University            Shiraz, Iran
	Villanova, Pennsylvania 19085

References:

Hall, D. S., (1976), IAU Colloq. No. 29, part 1. p. 287. [BIBCODE 1976ASSL...60..287H ]
Rigterink, P. V., (1972), Astron. J., 77, 230. [BIBCODE 1972AJ.....77..230R ]
Szafraniec, R., (1948), Acta Astron., 4, Series C, 81.