Optical and ROSAT X-ray observations of the dwarf nova OY Carinae in superoutburst and quiescence

We present ROSAT X-ray and optical light curves of the 1994 February supero utburst of the eclipsing SU UMa dwarf nova OY Carinae. There is no eclipse of the flux in the ROSAT HRI light curve. Contemporaneous 'wide B'-band opt ical light curves show extensive superhump activity and dips at superhump m aximum. Eclipse mapping of these optical light curves reveals a disc with a considerable physical flare, even three days into the superoutburst declin e. We include a later (1994 July) ROSAT PSPC observation of OY Car that allows us to put constraints on the quiescent X-ray spectrum. We find that while there is little to choose between OY Car and its fellow high-inclination sy stems with regard to the temperature of the emitting gas and the emission m easure, we have difficulties reconciling the column density found from our X-ray observation with the column found in Hubble Space Telescope UV observ ations by Horne et al, The obvious option is to invoke time variability.