Post common envelope binary

A post-common envelope binary (PCEB) or pre-cataclysmic variable is a <a href="/facts/Binary_system/9aRPc441">binary system</a> consisting of a <a href="/facts/White_dwarf/gIYlTtrC">white dwarf</a> or <a href="/facts/Hot_subdwarf/5h8hyTHM">hot subdwarf</a> and a <a href="/facts/Main-sequence_star/DJ3Wq8kd">main-sequence star</a> or a <a href="/facts/Brown_dwarf/fa7kr9lH">brown dwarf</a>. The star or brown dwarf shared a <a href="/facts/Common_envelope/0HDSs6ME">common envelope</a> with the white dwarf progenitor in the <a href="/facts/Red_giant/lla6Bsvy">red giant</a> phase. In this scenario the star or brown dwarf loses <a href="/facts/Angular_momentum/7TuSVFxq">angular momentum</a> 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 <a href="/facts/Magnetic_braking_(astronomy)/pCcYRYTl">magnetic braking</a> and <a href="/facts/Gravitational_wave/4XWN3NtG">gravitational waves</a> and will eventually begin mass-transfer, resulting in a <a href="/facts/Cataclysmic_variable_star/QgRCfaYj">cataclysmic variable</a>. While there are thousands of PCEBs known, there are only a few <a href="/facts/Eclipsing_binaries/TYlDe1bD">eclipsing</a> 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 <a href="/facts/Jupiter_mass/EtLUmeEN">MJ</a> might <a href="/facts/Photoevaporation/0VntLhcD">evaporate</a> during the common envelope phase and therefore the secondary is supposed to have a mass higher than 20 MJ.
The material ejected from the common envelope forms a <a href="/facts/Planetary_nebula/5Vy84cPq">planetary nebula</a>. One in five planetary nebulae are 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 <a href="/facts/HD_101584/v1MCm1cg">HD 101584</a> 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 <a href="/facts/Eclipsing_binary/TYlDe1bD">eclipsing</a> post-common envelope binaries show variations in the timing of eclipses, the cause of which is uncertain. While orbiting <a href="/facts/Exoplanet/zjfmqfUf">exoplanets</a> are often proposed as the cause of these variations, planetary models often fail to predict subsequent changes in eclipse timing. Other proposed causes, such as the <a href="/facts/Applegate_mechanism/88DhCzY6">Applegate mechanism</a>, often cannot fully explain the observed eclipse timing variations either.

Post common envelope binary open-in-new

Post common envelope binary