Synchronization-induced period gaps and ultra-short periods in magnetic cataclysmic binaries

When synchronization occurs in magnetic cataclysmic variables, the spin angular momentum of the white dwarf primary is injected directly into the binary system by the magnetohydrodynamic torque. The authors show that this process brings the system out of contact and produces a new kind of period gap. If synchronization occurs when the orbital period Pb ⪉ 1.5 hr, the secondary is degenerate when contact is reestablished and the orbital period can be as short as 25 minutes. Thus binaries with hydrogen-rich companion stars can form with periods much shorter than the usual minimum period of ≡72 minutes.