Far-Ultraviolet FUSE Observations of the Dwarf Novae SS Aurigae and RU Pegasi in Quiescence

We have analyzed the Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of two U Geminorum type dwarf novae, SS Aurigae and RU Pegasi, observed 28 days and 60 days (respectively) after their last outburst. In both systems, the FUSE spectra (905–1182 Å) reveal evidence of the underlying accreting white dwarf exposed in the far-UV. Our grid of theoretical models yielded best-fitting single-temperature photosphere models to the FUSE spectra with Teff = 33,000 K for SS Aur and Teff = 53,000 K for RU Peg. For SS Aur, a white dwarf of 27,000 K plus a 48,000 K accretion belt yields the best agreement with the FUSE spectrum and parallax distance. This work provides two more dwarf nova systems with known white dwarf temperatures above the period gap where few are known. The absence of C III (1175 Å) absorption in SS Aur and the elevation of N above solar suggests the possibility that SS Aur represents an additional accreting white dwarf where the surface C/N ratio derives from CNO processing. For RU Peg, the modeling uncertainties prevent any reliable conclusions about the surface abundances and rotational velocity.