Proper Motion Selected White Dwarf -- Red Dwarf Binaries

Abstract

One of the main unknowns in binary evolution is the so-called common-envelope phase: a short lived phase during which the core of a giant and the companion star exist within a single envelope. Products and good tracers of common-envelope evolution are detached close white dwarf + low-mass main-sequence star (red dwarf) binaries. Their combined colours locate them in distinct regions of colour-colour diagrams. We have used a combination of proper motion and SDSS colours to distinguish between our targets and background quasars. We selected a very clean population of white dwarf -- red dwarf binaries. We will investigate the orbital period distribution of these systems together with their characteristics to compare with results from population synthesis codes and set limits on the physics of the common-envelope phase