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 , \dot { M } , is increasing , gravity wave ( GW ) emission continues to drive the binary to shorter orbital period , P _ { \mathrm { orb } } .
We argue that the phase where \dot { M } > 0 and \dot { \nu } > 0 ( \nu = 1 / P _ { \mathrm { orb } } ) can last between 10 ^ { 3 } and 10 ^ { 6 } yrs , significantly longer than previously estimated .
These results are applied to RX J0806+1527 ( P _ { \mathrm { orb } } = 321 s ) and RX J914+2456 ( P _ { \mathrm { orb } } = 569 s ) , both of which have measured \dot { \nu } > 0 . Thus , a \dot { \nu } > 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 \ddot { \nu } \approx 1.0 - 1.5 \times 10 ^ { -28 } Hz s ^ { -2 } and argue that timing observations can probe \ddot { \nu } at this level with a total \approx 20 yr baseline .
We also place constraints on each system ’ s initial parameters given current observational data .