post-common-envelope binary

A post-common-envelope binary (PCEB) is a binary star with particular characteristics that do not fit within the typical stellar evolution of independent main sequence stars. The particular characteristics are presumed to indicate a history including a common envelope (CE) phase, a phase during which mass falling from one star to the other expands into a gas cloud that surrounds them both, i.e., an envelope common to both stars. A star in such a phase is called a common-envelope binary (CEB) and the phase is called a common-envelope-binary event (CEBE), the word event used because the phase is assumed to be relatively short.

Such common-envelope-binary events are deduced to explain the above-mentioned unusual characteristics, which are considered plausible because some observed main-sequence binary pairs are positioned so that one will obviously evolve into a giant star large enough to encompass the other, and mass transfer would be inevitable. Binary stars are thought to generally form in pairs (simultaneously) and the characteristics of sufficiently-independent stars should reflect this identical age along with their mass, as per the Vogt-Russell theorem. An occurrence of mass transfer between binaries explains Algol, a star system with such a mismatched pair of stars. I believe Algol is not considered a post-common-envelope binary, but models explaining some other binary systems incorporate a common-envelope phase in their history. Such a common-envelope phase might end with the two stars coalescing, or with the envelope blown away, or leaking away. In the latter cases, a post-common-envelope binary is the result.