While the majority of the wide main sequence binaries evolve as if
they were single stars and never interact, a small percentage are
believed to be the progenitors of close compact binaries. In this
scenario, the earlier main sequence star becomes a red giant and
eventually overfills its Roche lobe. Mass transfer is then initiated
and the unstable in-falling material engulfs the main sequence
companion and leads it to overfill its own Roche lobe. The two stars
orbit inside a common envelope, and the friction
inside it drives to a rapid decrease of the binary separation. The
energy and angular momentum extracted from the binary orbit ultimately
eject the envelope. When the envelope is expelled the resulting
post-CE binary continues decreasing its orbital separation through
angular momentum loss, mainly via magneting braking (MB).

Both CE and MB are known for long but still poorly understood, and
significant progress will only be achieved if they can be calibrated
using innovative observational input. Post-CE binaries are probably
among the best-suited class of objects to improve our understanding of
close binary evolution, because (1) they are both numerous and
well-understood in terms of their stellar components, and (2) they are
not contaminated by the presence of an accretion disc. The Sloan
Digital Sky Survey (SDSS) is currently providing the possibility of
dramatically increasing the size of the sample of known PCEBs, with
already more than 1200 white dwarf-main sequence binaries (WDMS)
identified. I am involved in a program
dedicated to identify and measure orbital periods of SDSS PCEBs.
This new, large sample of well-studied
PCEBs will then provide the much-needed constraints for further
development of binary evolution theory.