Period Changes in Ultra-compact Double White Dwarfs

In recent years there has been much interest in the nature of two stars, V407 Vul and RX J0806+1527, which are widely thought to be binary white dwarfs of very short orbital period, 570 and 321 seconds respectively. As such they should be strong sources of gravitational waves and possible ancestors of the accreting AM CVn stars. Monitoring at X-ray and optical wavelengths has established that the period of each star is decreasing, at rates compatible with that expected from gravitational radiation. This has been taken to support the “unipolar inductor” model in which the white dwarfs are detached and the X-rays produced by the dissipation of magneticallyinduced electric currents. In this paper we show that this interpretation is incorrect because it ignores associated torques which transfer angular momentum between the spin of the magnetic white dwarf and the orbit. We show that this torque is ∼ 10 times larger than the GR term in the case of V407 Vul and ∼ 10 times larger for RX J0806+1527. For V407 Vul, the unipolar inductor model can only survive if the white dwarf spins ∼ 100 times faster than the orbit. Since this could only come about through accretion, the validity of the unipolar inductor appears questionable for this star. We also consider whether accretion models can fit the observed spinup, concluding that they can, provided that a mechanism exists for driving the mass transfer rate away from its equilibrium value.