  We present the first white dwarf mass distributions of a large and
homogeneous sample of post-common envelope binaries (PCEBs) and wide white
dwarf-main sequence binaries (WDMS) directly obtained from observations. Both
distributions are statistically independent, with PCEBs showing a clear
concentration of systems towards the low-mass end of the distribution, and the
white dwarf mass distribution of wide WDMS binaries being similar to those of
single white dwarfs. Our results provide evidence that the majority of low-mass
(Mwd < 0.5Msun) white dwarfs are formed in close binaries.
