2006ApJ...649L..99D

We report on evolutionary calculations of the onset of mass transfer in AM CVn binaries, treating the donor's evolution in detail. We show that during the early contact phase, while the mass transfer rate, M{dot}, is increasing, gravity wave emission continues to drive the binary to shorter orbital period, P_orb. We argue that the phase where M{dot}>0 and ν{dot}>0 (ν=1/P_orb) can last between 10³ and 10⁶ yr, significantly longer than previously estimated. These results are applied to RX J0806+1527 (P_orb=321 s) and RX J914+2456 (P_orb=569 s), both of which have measured ν{dot}>0. Thus, aν{dot}>0does not select between the unipolar inductor and accretion-driven models proposed as the source of X-rays in these systems. For the accretion model, we predict for RX J0806 that ν{ddot}~(1.0-1.5)x10^–28 Hz/s2 and argue that timing observations can probe ν{ddot} at this level with a total ~20 yr baseline. We also place constraints on each system's initial parameters given current observational data.