A post-common-envelope binary (PCEB) or pre-cataclysmic variable is a binary system consisting of a white dwarf or hot subdwarf and a main-sequence star or a brown dwarf. The star or brown dwarf shared a common envelope with the white dwarf progenitor in the red-giant phase. In this scenario, the star or brown dwarf loses angular momentum as it orbits within the envelope, eventually leaving a main-sequence star and white dwarf in a short-period orbit. A PCEB will continue to lose angular momentum via magnetic braking and gravitational waves and will eventually begin mass transfer, resulting in a cataclysmic variable. While there are thousands of PCEBs known, there are only a few eclipsing PCEBs, also called ePCEBs. Even more rare are PCEBs with a brown dwarf as the secondary. A brown dwarf with a mass lower than 20 might evaporate during the common-envelope phase, so the secondary is supposed to have a mass higher than 20 .
The material ejected from the common envelope forms a planetary nebula. One in five planetary nebulae are thought to be ejected from common envelopes, but this might be an underestimate. A planetary nebula formed by a common-envelope system usually shows a bipolar structure.
The suspected PCEB HD 101584 is surrounded by a complex nebula. During the common-envelope phase, the red-giant phase of the primary was terminated prematurely, avoiding a stellar merger. The remaining hydrogen envelope of HD 101584 was ejected during the interaction between the red giant and the companion, and it now forms the circumstellar medium around the binary.
Many eclipsing post-common-envelope binaries show variations in the timing of eclipses, the cause of which is uncertain. While orbiting exoplanets are often proposed as the causes of these variations, planetary models often fail to predict subsequent changes in eclipse timing. Other proposed causes, such as the Applegate mechanism, often cannot fully explain the observed eclipse timing variations either.
List of post-common envelope binaries Sorted by increasing orbital period. +NamePeriodSecondaryNoteSDSS J1205-024271.2 minuteslow-mass star or brown dwarfshortest period PCEB (as of 2017)WD 0137−349116 minutesbrown dwarffirst confirmed PCEB with a brown dwarf as a companionCSS21055121.73 minutesbrown dwarfeclipsing binarySDSS 15572.27 hoursbrown dwarfcircumbinary debris disk with a polluted white dwarfV470 Camelopardalis(HS0705+6700)red dwarfeclipsing binaryNY Virginisred dwarfeclipsing binaryNSVS 14256825red dwarfeclipsing binaryHW Virginisred dwarfeclipsing binaryNN Serpentis3.12 hoursred dwarfeclipsing binaryWD 0837+1854.2 hoursbrown dwarfextreme mass ratio of the progenitor, with the primary having a mass of 3.5-3.7 and the secondary 25-30 RR Caelired dwarfeclipsing binaryDE Canum Venaticorumred dwarfeclipsing binarycentral source of Hen 2-1114.616 hoursK-type main sequence starplanetary nebula and eclipsing binaryK 1-216.2192 hoursplanetary nebulacentral source of Fleming 11.1953 dayswhite dwarfplanetary nebulaKOI-2561.37865 daysred dwarfeclipsing binarycentral source of NGC 23921.9 dayshot white dwarfplanetary nebula and x-ray binarycentral source of NGC 51894.04 daysmassive white dwarfplanetary nebula; primary is a low-mass Wolf-Rayet starG 203-4714.91 daysred dwarfnearest PCEBcentral source of NGC 234616 days>3.5 sub-giantplanetary nebula; one of the longest period PCEB which could host the most massive secondaryHD 101584150–200 daysred dwarf or white dwarfthe engulfment of the companion probably triggered gas to outflow, creating the nebula, seen with Hubble and ALMA; primary is a post-RGB star
See also Cataclysmic variable Second generation planet
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