System parameters of three short-period cataclysmic variable stars

Abstract

ABSTRACT Using photometric ULTRACAM observations of three new short-period cataclysmic variables (CVs), we model the primary eclipse light curves to extract the orbital separation, masses, and radii of their component stars. We find donor masses of $0.060\pm 0.008\, {\rm M_\odot }$, $0.042\pm 0.001\, {\rm M_\odot }$, and $0.042\pm 0.004\, {\rm M_\odot }$, two being very low-mass sub-stellar donors, and one within 2σ of the hydrogen burning limit. All three of the new systems lie close to the empirical evolutionary sequence that has emerged from observations of the last decade. We briefly re-evaluate the long-standing discrepancy between observed donor mass and radius data, and theoretical CV evolutionary tracks. By looking at the difference in the observed period at each mass and the period predicted by the modelled evolutionary sequences, we qualitatively examine the form of excess angular momentum loss that is missing from the models below the period gap. We show indications that the excess angular momentum loss missing from CV models grows in importance relative to gravitational losses as the period decreases. Detailed CV evolutionary models are necessary to draw more quantitative conclusions in the future.