2015MNRAS.449..175S

Estimates of white dwarf radii from photometric observations of eclipsing cataclysmic variables critically depend on the shape of the emitting region which has traditionally been assumed to be spherical and unchanging. Derivative analysis of light curves can reveal the shape of the emitting boundary layer but requires high signal-to-noise ratio (SNR), fast photometry of the central, compact object. With this objective, 18 light curves were obtained from high time resolution observations of OY Car using the Southern African Large Telescopes (SALT) Imaging Camera (SALTICAM). The technique of total variation regularization, adapted and applied here for the first time to astronomical time series data, combined with the high SNR provided by SALTICAM, allowed analysis of the light curves on an individual basis. This showed that the shape of the emitting region does indeed vary significantly; the boundary layer emission changes on an orbital cycle-to-cycle basis. This implies that previous analyses have led to overestimates of the white dwarf's radius. Nearly all of the light curves presented here did not match previously proposed models for the boundary layer's surface brightness distribution.