2006MNRAS.369.1537S

We present high-quality optical spectroscopy of the SUUMa-subtype dwarf nova, VW Hyi taken while the system was in quiescence. An S-wave is executed by the emission cores of the hydrogen Balmer lines and by the emission lines of Hei, Caii, FeII and Heii. Using Doppler tomography, we show it originates in the accretion stream-disc impact region. The HeII emission is strongly phase-dependent, suggesting it originates exclusively within a hot cavity at the initial impact. We map the ionization structure of the stream-disc interaction region. One possible interpretation of this is that the Balmer hotspot lies downstream of the HeII hotspot in the outer accretion disc, with the Hei and CaII hotspots at intermediate locations between the two. This suggests that Balmer emission is suppressed until material has cooled somewhat downstream of the impact site and is able to recombine. We favour a phase offset of 0.15±0.04 between the photometric ephemeris and inferior conjunction of the mass donor. The white dwarf contributes significantly to the optical continuum, with broad Balmer absorption and narrow Mgiiλ4481 absorption clearly apparent. This latter feature yields the gravitational redshift: v_grav= 38±21 km/s, so M₁= 0.71^+0.18_–0.26 M_☉. This implies M₂= 0.11±0.03 M_☉and hence the donor is not a brown dwarf. A prominent Balmer jump is also observed. We note that the previously accepted system parameters for both VW Hyi and WXHyi incorporate an algebraic error, and we provide a recalculated M₁(q) plane for WXHyi.