  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)<=300km/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.
