X-ray and optical observations of the dwarf nova HT Cas, some simultaneous, are presented. The X-ray light curve shows that the X-ray source is eclipsed and comes from the near vicinity of the white dwarf. It must therefore be a boundary layer. The X-ray flux (0.1−1.2 keV) is |~5×10⁻¹³erg s⁻¹|. The spectrum is consistent with an absorbed thermal bremsstrahlung model with temperature |KT~2.4| keV and hydrogen column density |n_ H~1.8×10²⁰ cm⁻²|. The low luminosity in the boundary layer, |L~5×10³⁰erg s⁻¹|, may be caused by having a low mass-transfer rate on to the white dwarf, of |~1×10⁻¹² M_ ⁻¹|, or perhaps the presence of a rapidly rotating white dwarf. The optical observations show that HT Cas has low states where the accretion rate in the disc is dramatically reduced from the usual quiescent level. The drop from a normal to a low state can occur in less than two weeks. The measured white dwarf fluxes in the low state show that the white dwarf has a temperature of ∼ 13200 K, cooler than the temperature determined from the normal state of 18700 K. The cooling of the white dwarf must occur in less than two weeks. The ROSAT observations were obtained during a low state.
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Wood et al. (1995) studied this question.