2009ApJ...705..693S

Binary evolution predicts a population of helium core (M < 0.5 M_☉) 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 {dot}M<10^{-10} M_☉\yr^{-1} set by angular momentum losses. We show here that the cold He core temperatures (T_c< 10⁷ K) and low {dot}M result in ∼10^–3 M_☉ 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_☉, 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 1E 0035.4-7230 (P_orb= 4.1 hr).