2005ApJ...629..451G

We have observed the short-period dwarf novae BW Scl, BC UMa, and SW UMa using the Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS). In all three systems, the white dwarf is the dominant source of far-ultraviolet flux, even though in BC UMa and SW UMa an additional continuum component contributes ∼10% and ∼20% of the 1400 Å flux, respectively. Fitting the data with detailed white dwarf model spectra, we determine the effective temperatures to be 14,800±900 K (BW Scl), 15,200±1000 K (BC UMa), and 13,900±900 K (SW UMa). The additional continuum component in BC UMa and SW UMa is equally well described by either a blackbody or a power law, which could be associated with emission from the hot spot or from an optically thin accretion disk (or an optically thin layer on top of a colder optically thick disk), respectively. Modeling the narrow metal lines detected in the STIS spectra results in subsolar abundances of carbon, oxygen, and silicon for all three systems and also suggests substantial suprasolar abundances of aluminum. The narrow absorption line profiles imply low white dwarf rotation rates, vsini≲300 km/s for the three white dwarfs. SW UMa is the only system that shows significant short-term variability in the far-ultraviolet range, which is primarily associated with the observed emission lines.