DETECTION OF THE SECONDARY STAR IN HT CASSIOPEIAE

Low-resolution spectra of the eclipsing dwarf nova HT Cas, taken over the range 5000-9800 Å show TiO bands and Na I absorption lines from the secondary star. From the TiO band ratio at mid-eclipse we estimate that the secondary star contributes ≈ 37% of the light at 7500 Å during the eclipse and that it has a Boeshaar spectral type M5.4 ± 0.3. The mass, radius, and luminosity of the secondary star are all consistent with mainsequence values. The TiO band strength diminishes greatly near phase 0.5, but an eclipse by the disk is not sufficient to explain this. The same behavior has been seen in Z Cha, and again it shows that the TiO bands are weaker on the side facing the white dwarf. We are able to measure the radial velocity from the sodium line near 8200 A for most of the spectra and after a correction for the asymmetric distribution of absorption over the secondary star, we find its radial velocity semiamplitude K2 = 430 ± 25 km s-1. For this value of K2 and for a range of possible mass ratios q = 0.10 to 0.40, we estimate that the distance to HT Cas lies between 120 and 230 pc, with the uncertainty dominated by the uncertainty in q. Our distance conflicts with the estimates from light curve fits by Zhang et al. and from infrared data by Berriman et al. We believe that the assumption by Zhang et al. that the accretion disk is optically thick is incorrect but are unable to reconcile our data with the result by Berriman et al.