On the evolutionary status of short period cataclysmic variables

Abstract

We present system parameters for a small sample of short period, eclipsing CVs taken from the Sloan Digital Sky Survey. Three out of seven of the systems possess brown dwarf donor stars and have thus evolved past the orbital period minimum. This is broadly consistent with predictions that 40–70 per cent of CVs should have evolved past the orbital period minimum. The donor star masses themselves are inconsistent with model predictions, unless enhanced angular momentum loss (e.g. from circumbinary discs) is invoked. However, the mass transfer rates as deduced from white dwarf effective temperatures are not consistent with enhanced mass transfer rates. Solution of this conundrum will require independent estimates of the mass transfer rates in short period CVs. The white dwarfs show a strong tendency towards high masses. This may in part be due to selection effects. There is one Iron core white dwarf, and no Helium core white dwarfs in our sample, despite predictions that 30–80 per cent of short period CVs should contain Helium core white dwarfs.