Ultrashort-Period Binaries. II. HZ 29 (=AM CVn): a Double-White Semidetached Postcataclysmic Nova?

Abstract

The peculiar white dwarf HZ 29 is in fact an eclipsing binary system with P∼1051.05 secs (∼ 17.5 mins). A hydrogen-containing secondary is most unlikely. A satisfactory model has a degenerate helium secondary of mass ∼0.041 M⊙. We suggest this is the remnant core of a nova secondary which has suffered complete envelope mass loss as a result of gravitational radiation angular momentum losses. The separation of components is 10^10 cm, primary orbital velocities appear to be marginally detectable, and the period should be increasing by 1 or 2 parts in 10^8 per annum.