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 M _ { \odot } .
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 , 1.7 \times 10 ^ { -4 } yr ^ { -1 } , 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 .