  We use multi-epoch spectroscopy of about 4000 white dwarfs in the Sloan
Digital Sky Survey to constrain the properties of the Galactic population of
binary white dwarf systems and calculate their merger rate. With a Monte Carlo
code, we model the distribution of DRVmax, the maximum radial velocity shift
between exposures of the same star, as a function of the binary fraction within
0.05 AU, fbin, and the power-law index in the separation distribution at the
end of the common envelope phase, alpha. Although there is some degeneracy
between fbin and alpha, the the fifteen high DRVmax systems that we find
constrain the combination of these parameters, which determines a white dwarf
merger rate per unit stellar mass of 1.4(+3.4,-1.0)e-13 /yr/Msun (1-sigma
limits). This is remarkably similar to the measured rate of Type Ia supernovae
per unit stellar mass in Milky-Way-like Sbc galaxies. The rate of
super-Chandrasekhar mergers is only 1.0(+1.6,-0.6)e-14 /yr/Msun. We conclude
that there are not enough close binary white dwarf systems to reproduce the
observed Type Ia SN rate in the 'classic' double degenerate super-Chandrasekhar
scenario. On the other hand, if sub-Chandrasekhar mergers can lead to Type Ia
SNe, as recently suggested by some studies, they could make a major
contribution to the overall Type Ia SN rate. Although unlikely, we cannot rule
out contamination of our sample by M-dwarf binaries or non-Gaussian errors.
These issues will be clarified in the near future by completing the follow-up
of all 15 high DRVmax systems.
