Post-common envelope binaries from SDSS-X: the origin of low-mass white dwarfs

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 (WDMS) binaries 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 that of single white dwarfs. Our results provide evidence that the majority of low-mass (M-wd less than or similar to 0.5 M-circle dot) white dwarfs are formed in close binaries.