Superhumps and ultraviolet superdips: HST observations of OY Car

HST observations of the cataclysmic variable OY Car in superoutburst show dips in its ultraviolet light curve coincident with the optical superhump. By analogy with the light curves of X-ray dippers, we believe this to be due to obscuration of the hottest regions of the disc by the vertical structure flaring up at the edge of the disc. We present a new method for deducing the vertical structure of accretion discs surrounding X-ray dippers and cataclysmic variables. With a non-linear maximum entropy technique we are able to compute simultaneously the flux distribution from the surface of the disc, and the vertical and azimuthal extent of flaring at the edge of the disc. We apply this method to our observations of OY Car in the ultraviolet, and deduce the wavelength-dependence of the obscuration. These results are used to confirm our model of superhumps in cataclysmic variables as time-dependent changes in the thickness of the disc at its outer edge which then obscure ultraviolet flux and reprocess it into the optical part of the spectrum.