Hubble Space TelescopeSTIS Observations of the Accreting White Dwarfs in BW Sculptoris, BC Ursae Majoris, and SW Ursae Majoris

We have observed the short-period dwarf novae BW Scl, BC UMa and SW UMa using the Hubble Space Telescope/Space Telescope Imaging Spectrograph. 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 1400A flux, respectively. Fitting the data with detailed white dwarf model spectra, we determine the effective temperatures to be 14800+-900K (BW Scl), 15200+-1000K (BC UMa), and 13900+-900K (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. Modelling the narrow metal lines detected in the STIS spectra results in sub-solar abundances of carbon, oxygen and silicon for all three systems, and also suggests substantial supra-solar abundances of aluminium. The narrow absorption line profiles imply low white dwarf rotation rates, v sin(i)