ECLIPSING BINARIES IN THE OGLE VARIABLE-STAR CATALOG .2. LIGHT CURVESOF THE W-UMA-TYPE SYSTEMS IN BAADE WINDOW

Light curves of the contact systems visible in the direction of Baade' s Window have been analyzed using the first coefficients of the Fourie r representation. The results confirm that the geometric contact betwe en components is usually weak. Systems showing significant differences in the depths oi. eclipses are very rare in the volume-limited sample to 3 kpc: only two among 98 contact systems show the difference large r than 0.065 mag; for most systems the difference is <0.04 mag. If thi s relative frequency of 1/50 is representative, then one among 12,500- 15000 Main Sequence F-K spectral-type stars is either a semi-detached or poor-thermal-contact system. Below the orbital period of 0.37 day, no systems with appreciable differences in the eclipse depths have bee n discovered. Since large depth differences are expected to be associa ted with the ''broken-contact'' phase of the Thermal Relaxation Oscill ations, this phase must be very short for orbital periods above 0.37 d ay and possibly entirely absent for shorter periods. In the full sampl e, which is dominated by intrinsically bright, distant, long-period sy stems, larger eclipse-depth differences are more common with about 9% of binaries showing this effect. Sizes of these differences correlate with the sense of light-curve asymmetries (differing heights of maxima ) for systems with orbital periods longer than 0.4 day suggesting an a dmixture of semi-detached systems with accretion hot spots on cooler c omponents. The light-curve amplitudes in the full sample as well as in its volume-limited sub-sample are surprisingly small and strongly sug gest a mass-ratio distribution steeply rising toward more dissimilar c omponents. Since the sky-held sample is dominated by contact binaries with large amplitudes, it is suggested that a large fraction of low ma ss-ratio systems remains to be discovered among bright stars in the sk y. For a mass-ratio distribution emphasizing low values, an approach b ased on the statistics of the inner contact angles of totally eclipsin g systems may offer a better means of determining this distribution th an the statistics of the variability amplitudes. (C) 1997 American Ast ronomical Society.