  We present Chandra and Swift X-ray observations of four extremely low-mass
(ELM) white dwarfs with massive companions. We place stringent limits on X-ray
emission from all four systems, indicating that neutron star companions are
extremely unlikely and that the companions are almost certainly white dwarfs.
Given the observed orbital periods and radial velocity amplitudes, the total
masses of these binaries are greater than 1.02 to 1.39 Msun. The extreme mass
ratios between the two components make it unlikely that these binary white
dwarfs will merge and explode as Type Ia or underluminous supernovae. Instead,
they will likely go through stable mass transfer through an accretion disk and
turn into interacting AM CVn. Along with three previously known systems, we
identify two of our targets, J0811 and J2132, as systems that will definitely
undergo stable mass transfer. In addition, we use the binary white dwarf sample
from the ELM Survey to constrain the inspiral rate of systems with extreme mass
ratios. This rate, 0.00017/year, is consistent with the AM CVn space density
estimated from the Sloan Digital Sky Survey. Hence, stable mass transfer double
white dwarf progenitors can account for the entire AM CVn population in the
Galaxy.
