Hubble Space Telescope/STIS Spectroscopy of the White Dwarfs in the Short-Period Dwarf Novae LL Andromedae and EF Pegasi

Abstract

We present new Hubble Space Telescope Space Telescope Imaging Spectrograph observations of the short-period dwarf novae LL And and EF Peg during deep quiescence. We fit stellar models to the UV spectra and use optical and IR observations to determine the physical parameters of the white dwarfs in the systems, the distances to the binaries, and the properties of the secondary stars. Both white dwarfs are relatively cool, having T_eff near 15,000 K, and consistent with a mass of 0.6M_solar. The white dwarf in LL And appears to be of solar abundance or slightly lower, while that in EF Peg is near 0.1-0.3 times solar. LL And is found to be 760 pc away, while EF Peg is closer, at 380 pc. EF Peg appears to have an ~M5 V secondary star, consistent with that expected for its orbital period, while the secondary object in LL And remains a mystery. Based on observations made with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the AURA under NASA contract NAS 5-26555 and with the Apache Point Observatory (APO) 3.5 m telescope, which is operated by the Astrophysical Research Consortium (ARC).