Accretion-disk precession and substellar secondaries in cataclysmic variables

The mass-losing secondaries in cataclysmic binaries are progressively whitt led away by the ongoing loss of angular momentum. The expected rate of evol ution implies that the binaries should spend most of their lives at very sh ort orbital period, with light secondaries (<0.08 M-.). But compared to the nearby white M dwarf and accretion disk, these secondaries are effectively dark, so it has been quite difficult to learn anything about them from obs ervation. 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 actua l 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 at 0.08 M-., the secondari es are significantly larger. This bloating, M possibly due to an extra angu lar momentum sink in the binary, sets the value of the minimum orbital peri od for H-rich binaries to be 76-80 minutes. Seven stars are found with seco ndaries in the mass range 0.014-0.06. M-..