Found: The Progenitors of AM CVn and Supernovae .Ia
  We present optical and X-ray observations of two tidally distorted, extremely
low-mass white dwarfs (WDs) with massive companions. There is no evidence of
neutron stars in our Chandra and XMM observations of these objects. SDSS
J075141.18$-$014120.9 (J0751) is an eclipsing double WD binary containing a
0.19 Msol WD with a 0.97 Msol companion in a 1.9 h orbit. J0751 becomes the
fifth 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 > 1.11
Msol WD companion in a 1.5 h orbit. With a mass ratio of ~0.1, J1741 will have
stable mass transfer through an accretion disk and turn into an interacting AM
Canum Venaticorum (AM CVn) system in the next ~160 Myr. With a mass ratio of
0.2, J0751 is likely to follow a similar evolutionary path. These are the first
known AM CVn progenitor binary systems and they provide important constraints
on the initial conditions for AM CVn. Theoretical studies suggest that both
J0751 and J1741 may create thermonuclear supernovae in ~10^8 yr, either .Ia or
Ia. Such explosions can account for ~1% of the Type Ia supernova rate.
