Visual and infrared photometry of the ultrashort-period dwarf nova HT Cassiopeiae

This paper presents simultaneous visual (V) and infrared (H) photometry of the eclipsing dwarf nova binary HT Cas during two eclipses in October 1985, and infrared (JHK) photometry out of eclipse in January 1986. These are the first infrared observations of this object. They require that the red dwarf is fainter than 0.5 mJy at H(H = 15.7), and that the system is at least 215 pc away, for a red dwarf radius of 0.15 solar radii. At this distance, the white dwarf has a brightness temperature at V of T(b) = 26,000 K. The accretion disk around the white dwarf is highly variable from epoch to epoch: it was 70 percent brighter at H in January 1986 than in October 1985. Such variability in a quiescent system is unprecedented. The disk consisted of optically thin material in January, when it had a kinetic temperature of 10,000-20,000 K, and was most likely optically thin in October too. (Author)