On the formation of neon-enriched donor stars in ultracompact X-ray binaries

We study the formation of neon-enriched donor stars in ultracompact X-ray binaries (P-orb < 80 min) and show that their progenitors have to be low-mass (0.3-0.4) M. "hybrid" white dwarfs (with CO cores and thick helium mantles). Stable mass transfer is possible if in the initial stages of mass exchange mass is lost from the system, taking away the specific orbital angular momentum of the accretor ("isotropic re-emission"). The excess of neon in the transferred matter is due to chemical fractionation of the white dwarf which has to occur prior to the Roche lobe overflow by the donor. The estimated lower limit of the orbital periods of the systems with neon-enriched donors is close to 10 min. We show that the X-ray pulsar 4U 1626-67, which likely also has a neon-enriched companion, may have been formed via accretion induced collapse of an oxygen-neon white dwarf accretor if the donor was a hybrid white dwarf.