The eclipsing dwarf nova HS 1804+6753

We present observations of the recently discovered eclipsing dwarf nova HS 1804 + 6753. Our studies of the H alpha emission from this system show that it is dominated by emission from the bright spot and inner Lagrangian regions. The Doppler image of H alpha shows that the line emission from the bright spot is intrinsically broad, and this may explain why radial velocity measurements of the emission lines of cataclysmic variable stars are so often distorted. The accretion disc is found to be only 0.4R(L1) in radius and emission from it is weak, so HS 1804 + 6753 does not show the classic double-peaked line profile associated with accretion discs. The line emission from the bright-spot region has the velocity of the gas stream and is strong enough that its eclipse can be detected. From the disappearance of secondary star absorption features at phase 0.5, and distortions of the radial velocity curve at the same phase, we find evidence for significant radiation of the inner face of the secondary star by the white dwarf. Correcting for this, we find a radial velocity semi-amplitude of K-R = 210 +/- 14 km s(-1) for the secondary star. We also measure a rotational broadening of 140 +/- 10 km s(-1), consistent with a mass ratio of 0.8 based upon earlier measurements of the white dwarf's radial velocity. The secondary star is of spectral type M1 to M2 and contributes 20 to 30 per cent of the light at H alpha out of eclipse, and almost all of the light in eclipse.