MULTICOLOR PHOTOMETRY AND ECLIPSE MAPPING OF OY CARINAE IN A SUPEROUTBURST

The SU UMa type dwarf nova OY Car was observed in UBV and white light during its April 1992 superoutburst. The light curves show superhumps with a period of 0.06454 d, consistent with previous observations. The structure of the superhump varies from night to night and is also wav elength-dependent. Maximum-entropy eclipse mapping techniques are used to obtain maps of the surface brightness distribution of the accretio n disk. The disk maps show structures in their outer parts which can b e interpreted as the superhump light source. They are compatible with theoretical predictions and recent modelling of the superhumps. They p ersist until two nights before the end of the superoutburst, while dur ing the last night the bright spot of OY Car reappears at a location w hich suggests that the disk has shrunken below its quiescent radius. A two-color diagram of the pixels of the reconstructed disk shows that the inner disk is optically thick, with colors in between the blackbod y and main sequence relationships. The angular scale of the disk and t he distance to OY Car are estimated from a color-magnitude diagram by a method similar to cluster main-sequence fitting. We fit the Barnes-E vans relation to the pixels in the inner disk to find a distance of 86 +/- 4 pc. Temperatures in the disk range from similar to 6 000 K in t he outer regions to similar to 25 000 K near the white dwarf at disk c entre and closely follow the T proportional to R(-3/4) law for steady mass accretion at a rate of M=10(-9.20+/-0.08) M./year. The evolution of the temperature distribution during the decline from the superoutbu rst reveals the existence of a cooling front in the disk which slowly propagates inward.