HELIUM CORE WHITE DWARFS IN CATACLYSMIC VARIABLES

Binary evolution predicts a population of helium core (M < 0.5 M(circle dot))white dwarfs (WDs) that are slowly accreting hydrogen-rich material from low-mass main-sequence or brown dwarf donors with orbital periods less than 4 hr. Four binaries are presently known in the Milky Way that will reach such a mass-transferring state in a few Gyr. Despite these predictions and observations of progenitor binaries, there are still no secure cases of helium core WDs among the mass-transferring cataclysmic variables. This led us to calculate the fate of He WDs once accretion begins at a rate (M) over dot < 10(-10) M(circle dot) yr(-1) set by angular momentum losses. We show here that the cold He core temperatures (T(c) 10(7) K) and low (M) over dot result in similar to 10(-3) M(circle dot) of accumulated H-rich material at the onset of the thermonuclear runaway. Shara and collaborators noted that these large accumulated masses may lead to exceptionally long classical nova (CN) events. For a typical donor star of 0.2 M(circle dot), such binaries will only yield a few hundred CNe, making these events rare among all CNe. We calculate the reheating of the accreting WD, allowing a comparison to the measured WD effective temperatures in quiescent dwarf novae and raising the possibility that WD seismology may be the best way to confirm the presence of a He WD. We also find that a very long (> 1000 yr) stable burning phase occurs after the CN outburst, potentially explaining enigmatic short orbital period supersoft sources like RX J0537-7034 (P(orb) = 3.5 hr) and IE 0035.4-7230 (P(orb) = 4.1 hr).