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We present optical and X-ray observations of two tidally distorted, extremelylow-mass white dwarfs (WDs) with massive companions. There is no evidence ofneutron stars in our Chandra and XMM observations of these objects. SDSSJ075141.18$-$014120.9 (J0751) is an eclipsing double WD binary containing a0.19 Msol WD with a 0.97 Msol companion in a 1.9 h orbit. J0751 becomes thefifth eclipsing double WD system currently known. SDSS J174140.49+652638.7(J1741) is another binary containing a 0.17 Msol WD with an unseen M > 1.11Msol WD companion in a 1.5 h orbit. With a mass ratio of ~0.1, J1741 will havestable mass transfer through an accretion disk and turn into an interacting AMCanum Venaticorum (AM CVn) system in the next ~160 Myr. With a mass ratio of0.2, J0751 is likely to follow a similar evolutionary path. These are the firstknown AM CVn progenitor binary systems and they provide important constraintson the initial conditions for AM CVn. Theoretical studies suggest that bothJ0751 and J1741 may create thermonuclear supernovae in ~10^8 yr, either .Ia orIa. Such explosions can account for ~1% of the Type Ia supernova rate.

Found: The Progenitors of AM CVn and Supernovae .Ia