Epoch J2000.0 Equinox J2000.0
|Right ascension||03h 14m 13.03s|
|Declination||-22° 35′ 41.4″|
|Apparent magnitude (V)||14.5–17.3|
|Radial velocity (Rv)||70 km/s|
EF Eridani (abbreviated EF Eri, sometimes incorrectly referred to as EF Eridanus) is a variable star of the type known as polars, AM Herculis stars, or magnetic cataclysmic variable stars. Historically it has varied between apparent magnitudes 14.5 and 17.3, although since 1995 it has generally remained at the lower limit. The star system consists of a white dwarf star with a substellar massed former star in orbit.
EF Eridani B 
The substellar mass in orbit around the white dwarf is a star which lost all of its gas to the white dwarf, except what remains, a 0.05 solar mass ball, which is too small to continue fusion, and does not have the composition of a super-planet, brown dwarf, or white dwarf. There is no category for such a stellar remnant.
It is theorized that 500 million years ago, the white dwarf started to cannibalize its partner, when they were separated by 7 million km. As it lost mass, the regular star spiralled inward, until now they are separated by a mere 700,000 km.
See also 
- AM Herculis
- Cataclysmic variable star
- Nova-like star
- Polar (cataclysmic variable)
- Variable stars
- Stellar remnants
- PSR J1719-1438, a star system with a planetary-mass former star in orbit around a compact star
- PSR J1719-1438 b, a planetary-mass former star that was eroded by its binary star partner
- "V* EF Eri – Nova-like Star". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2009-12-17.
- "Query= EF Eri". General Catalogue of Variable Stars. Centre de Données astronomiques de Strasbourg. Retrieved 2009-12-17.
- Mystery Object Neither Star Nor Brown Dwarf
- Beuermann, K.; Wheatley, P.; Ramsay, G.; Euchner, F.; Gänsicke, B. T. (February, 2000). "Evidence for a substellar secondary in the magnetic cataclysmic binary EF Eridani". Astronomy and Astrophysics Letters 354: L49–L52. arXiv:astro-ph/0001183. Bibcode:2000A&A...354L..49B.
- Bailes, M.; Bates, S. D.; Bhalerao, V.; et al. (2011). "Transformation of a Star into a Planet in a Millisecond Pulsar Binary". Science 333 (6050): 1717–1720. arXiv:1108.5201. Bibcode:2011Sci...333.1717B. doi:10.1126/science.1208890. PMID 21868629.
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