Close binary evolution

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The evolution of close binary stars has three major unknowns: (a) How efficient is the common envelope phase in reducing the binary separation? (b) How strong is orbital angular momentum loss due to magnetic braking? (c) Does magnetic braking vanish if the secondary star is fully convective? Those questions can be answered by a thorough observationally study of white-dwarf/main-sequence binaries which have passed the common envelope phase in their binary evolution. The objects are abundant, although not yet known, they are sufficiently bright, the evolution of the binary components is reasonably well understood, and their light is not contaminated by accretion radiation. Based on archival SDSS data and own SEGUE spectroscopy we are generating a large data base of white-dwarf/main-sequence binaries, which harbour some 25% of close binaries of post-common envelope type. We are conducting a large-scale coordinated optical follow-up program to identify the close binaries, determine their orbital periods and perform a thorough statistical study to observationally constrain the magnetic braking law and to constrain the common envelope efficiency.