HIGH-PRECISION PHOTOMETRY OF THE RS-CVN BINARY-SYSTEM XY-UMA AND A DARK-ZONE PHOTOMETRIC MODEL

CCD V-band photometry is presented of the short-period RS CVn-type binary system XY UMa, providing a complete light curve defined by 1189 observations with a precision of 0.004 mag. The data were all obtained during 1992 October contemporaneously with ROS AT pointed-phase observations, and demonstrate that the primary star showed substantial star-spot activity at that time, predominantly on the hemisphere of the primary component that faces the cooler companion. Combining these accurate data with previously published spectroscopy, we obtain a good geometric description of the system, and are able to represent the distribution of surface brightness over the primary star by a dark-zone model centred on the equator, +/- 15-degrees in latitude, and encircling the star. The darkest part of that zone is near the substellar point. This representation is in very good agreement with that given by Bedford et al. for the coronal emission seen in X-rays by EXOSAT in 1986.