Accretion‐Disk Precession and Substellar Secondaries in Cataclysmic Variables

The mass-losing secondaries in cataclysmic,binaries are progressively whittled away by the ongoing loss of angular momentum.,The expected rateof evolution implies that the binaries should spend most of their lives at very short orbital period, with light secondaries (<0.08 M ). But compared to the nearby white dwarf and accretion disk, these secondaries are effectively dark; so it has been quite difficult to learn anything about them from observation. Here we show that for dwarf novae, the majority species, the mass ratios can be measured from simple photometric observations of “superhumps”, using equipment commonly possessed by amateur astronomers. The ,technique basically involves measuring ,the apsidal precession rate of the accretion disk, and thus has the merit of being purely dynamical, requiring no actual detection of light from the secondary. The results reaffirm what we have known,for a long time — that most secondaries are near the main sequence — but also show that near the end of the main sequence at0.08 M , the secondaries are significantly larger. This bloating, possibly due to an extra angular momentum sink in the binary, sets the value of the minimum orbital period for H-rich binaries to be 76–80 minutes. Seven stars are found with secondaries in the mass range 0.014– 0.06 M . Subject headings: accretion, accretion disks — binaries: close — novae, cataclysmic