PHOTOMETRIC EVIDENCE FOR WIDESPREAD SPOT ACTIVITY ON THE PRIMARY COMPONENT OF THE SHORT-PERIOD RS CVN BINARY XY UMA

A survey of published light curves, obtained since 1975, of the eclips ing RS CVn-type binary star XY UMa, coupled with results from a recent high-precision light curve, provides substantial evidence for the exi stence of a polar spot on the G2V primary component. The well-known an d strong variability of the shape of the light curve has been explaine d by the presence of surface maculation, often at low latitudes, which varies on time-scales of similar to 1 month, and which may persist fo r longer intervals. The 20-year span of published light curves, howeve r, shows that the entire system brightness varies by as much as 0.5 ma g in the V band. No orbital phase is exempt from this longer-term vari ation, suggesting that the portion of the dominant primary component t hat is always visible - namely the polar region - is at least partly r esponsible for the long-term variation in system brightness. It is pro posed that the surface of the primary component of XY UMa is very like that seen in the rapidly rotating single G dwarf star AB Dor, and the RS CVn binary DM UMa, namely, dominated by a polar spot but with a se cond belt of spot activity at lower latitudes.