On the evolutionary status of short period cataclysmic variables

TL;DR

This study uses high-speed photometry to identify and analyze short period cataclysmic variables, confirming the existence of post-period minimum systems and revealing discrepancies in donor star models.

Contribution

It provides the first observational evidence of the missing post-period minimum CV population and investigates donor star properties against theoretical predictions.

Findings

Three systems have brown dwarf donors, indicating evolved past the period minimum.

Donor stars are about 10% larger than models predict.

Mass transfer rates are inconsistent with enhanced angular momentum loss explanations.

Abstract

(Abridged) We present high-speed, three-colour photometry of seven short period (Porb <= 95 mins) eclipsing CVs from the Sloan Digital Sky Survey. We determine the system parameters via a parametrized model of the eclipse fitted to the observed lightcurve by chi2 minimization. Three out of seven of the systems possess brown dwarf donor stars and are believed to have evolved past the orbital period minimum. This is in line with predictions that 40-70 per cent of CVs should have evolved past the orbital period minimum. Therefore, the main result of our study is that the missing population of post-period minimum CVs has finally been identified. The donor star masses and radii are, however, inconsistent with model predictions; the donor stars are approximately 10 per cent larger than expected across the mass range studied here. One explanation for the discrepancy is enhanced angular momentum loss (e.g. from circumbinary discs), however the mass-transfer rates, as deduced from white dwarf effective temperatures, are not consistent with enhanced angular momentum loss. We show it is possible to explain the large donor radii without invoking enhanced angular momentum loss by a combination of geometrical deformation and the effects of starspots due to strong rotation and expected magnetic activity. Choosing unambiguously between these different solutions will require independent estimates of the mass-transfer rates in short period CVs.