A ROSAT OBSERVATION OF THE ECLIPSING BINARY-SYSTEM XY UMA

The short-period, eclipsing, magnetically active binary system XY UMa has been observed over several orbital cycles at X-ray wavelengths. Th e X-ray light curves vary owing to obvious flares and other lower leve l activity on time-scales of days. These data caution against deducing coronal structure on the basis of a single orbit of X-ray data. In co ntrast to similar binary systems, XY UMa shows no significant X-ray ec lipses. This is interpreted as evidence for either extended (> 1 R.) c oronae or a compact corona at high, uneclipsed latitudes on the primar y star. The extended coronal scenario is favoured by some observationa l features of other systems such as extended radio coronae, long-durat ion X-ray flares and cool prominences, but unfavoured by others such a s high coronal densities from EUV spectroscopy. A high-latitude compac t corona might be associated with the high-latitude starspots seen in many active stars. Nearly simultaneous optical light curves suggest th at some equatorial spots were present at the time of the X-ray observa tions, but fewer than in previous or subsequent years and that high-la titude spots must also be present. It is speculated that the lack of X -ray eclipses and dearth of equatorial, compact coronal regions are as sociated with a minimum in XY UMa's magnetic activity cycle.