The turn-on of mass transfer in AM CVn binaries: Implications for RX J0806+1527 and RX J1914+2456

We report on evolutionary calculations of the onset of mass transfer in AM CVn binaries, treating the donor's evolution in detail. During the early contact phase, while the mass transfer rate, Ṁ, is increasing, gravity wave emission continues to drive the binary to shorter orbital periods, P _orb. We show the phase where Ṁ > 0 and ν̇ > 0 (ν = 1/P _orb) can last between 10 ³ and 10 ⁶ yrs, significantly longer than previously estimated. We apply our calculations to RX J0806+1527 (P _orb = 321 s) and RX J914+2456 (P _orb = 569 s), both of which have measured ν̇ > 0, arguing that a ν̇ > 0 does 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 ν̈ ≈ 1.0 - 1.5 × 10 ^-28 Hz s ^-2 and argue that timing observations can probe ν̈ at this level with a total ≈ 20 yr baseline.