Pre-eclipse dips in the ultraviolet light curves of two eclipsing nova-like cataclysmic variables

Observations of the eclipsing nova-like variables UX UMa and RW Tri with the Hubble Space Telescope reveal substantial orbital modulations in their UV light. These modulations have a minimum about a quarter of an orbital cycle before eclipse. The modulation is deeper in RW Tri, which has the higher orbital inclination of the two binaries. The residual flux at minimum has a cooler spectral distribution than at maximum. In addition, the equivalent widths of various absorption lines in the UV spectrum decrease markedly towards the minimum of the modulation. Both of these facts suggest that the modulation is caused by the obscuration of the hot, inner regions of the accretion disc by cooler vertical structure towards the outside of the disc. This material must extend to a height above the disc which is >0.36 times its radius in the case of UX UMa which has i~70 deg. We draw attention to the published body of evidence of pre-eclipse optical dips in these CVs which also explicitly demands consideration of vertical structure. The phenomenon that we observe is reminiscent of the disc structure inferred in some low-mass X-ray binaries and outbursting cataclysmic variables. The present observations argue that such structure is also a feature of steady-state CV discs, and is probably a universal phenomenon in binary accretion discs.