  From a sample of spectra of 439 white dwarfs (WDs) from the ESO-VLT
Supernova-Ia Progenitor surveY (SPY), we measure the maximal changes in
radial-velocity (DRVmax) between epochs (generally two epochs, separated by up
to 470d), and model the observed DRVmax statistics via Monte-Carlo simulations,
to constrain the population characteristics of double WDs (DWDs). The DWD
fraction among WDs is fbin=0.100+/-0.020 (1-sigma, random) +0.02 (systematic),
in the separation range ~<4AU within which the data are sensitive to binarity.
Assuming the distribution of binary separation, a, is a power-law, dN/da ~
a^alpha, at the end of the last common-envelope phase and the start of solely
gravitational-wave-driven binary evolution, the constraint by the data is
alpha=-1.3+/-0.2 (1-sigma) +/-0.2 (systematic). If these parameters extend to
small separations, the implied Galactic WD merger rate per unit stellar mass is
R_merge=(1-80)e-13 /yr/Msun (2-sigma), with a likelihood-weighted mean of
R_merge=(7+/-2)e-13 /yr/Msun (1-sigma). The Milky Way's specific Type-Ia
supernova (SN Ia) rate is likely R_Ia~1.1e-13 /yr/Msun and therefore, in terms
of rates, a possibly small fraction of all merging DWDs (e.g. those with
massive-enough primary WDs) could suffice to produce most or all SNe Ia.
