A stellar prominence in the white dwarf/red dwarf binary QS Vir: evidence for a detached system

Using high resolution UVES spectra of the eclipsing Post Common Envelope Binary QS Vir we detect material along the line of sight to the white dwarf at orbital phase $\phi=0.16$. We ascribe this to a stellar prominence originating from the M dwarf secondary star which passes in front of the white dwarf at this phase. This creates sharp absorption features in the hydrogen Balmer series and Ca II H and K lines. The small size of the white dwarf allows us to place tight constraints on the column density of hydrogen in the n=2 level of log_(10)(N_2) = 14.10 +/- 0.03 cm^(-2) and, assuming local thermodynamical equilibrium, the temperature of the prominence material of ~9000K. The prominence material is at least 1.5 stellar radii from the surface of the M dwarf. The location of the prominence is consistent with emission features previously interpreted as evidence for Roche lobe overflow in the system. We also detect Mg II 4481A absorption from the white dwarf. The width of the Mg II line indicates that the white dwarf is not rapidly rotating, in contrast to previous work, hence our data indicate that QS Vir is a pre-cataclysmic binary, yet to initiate mass transfer, rather than a hibernating cataclysmic variable as has been suggested.