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. The X-ray light curves vary owing to obvious flares and other lower level 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 contrast to similar binary systems, XY UMa shows no significant X-ray eclipses. This is interpreted as evidence for either extended (> 1  R _⊙) coronae or a compact corona at high, uneclipsed latitudes on the primary star. The extended coronal scenario is favoured by some observational features of other systems such as extended radio coronae, long-duration X-ray flares and cool prominences, but unfavoured by others such as high coronal densities from EUV spectroscopy. A high-latitude compact corona might be associated with the high-latitude starspots seen in many active stars. Nearly simultaneous optical light curves suggest that some equatorial spots were present at the time of the X-ray observations, but fewer than in previous or subsequent years and that high-latitude spots must also be present. It is speculated that the lack of X-ray eclipses and dearth of equatorial, compact coronal regions are associated with a minimum in XY UMa's magnetic activity cycle.