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{{short description|Star in the constellation Eridanus}}
{{Starbox begin}}
{{Starbox image
| image = [[Image:EFEriLightCurve.png|250px]]
| caption = [[Photometric_system#Photometric_letters|Visual band]] [[light curve]]s for EF Eridani, adapted from Williams and Hiltner (1980)<ref name="Williams"/>
}}
{{Starbox observe
| epoch = [[J2000.0]]
| constell = [[Eridanus (constellation)|Eridanus]]
| ra = {{RA|03|14|13.25262}}<ref name="GaiaDR3"/>
| dec = {{DEC|-22|35|42.9001}}<ref name="GaiaDR3"/>
| appmag_v = 14.5–17.3<ref name=var />
}}
{{Starbox character
| class = DA<ref name=vincent2024/>
| variable = [[eclipsing binary|E]] + [[AM Herculis star|XM]]<ref name=var>{{cite web | title=Query= EF Eri
| url=http://www.sai.msu.su/gcvs/cgi-bin/search.cgi?search=EF+Eri
| publisher=[[Centre de Données astronomiques de Strasbourg]]
| work=[[General Catalogue of Variable Stars]] | access-date=2009-12-17}}</ref>
}}
{{Starbox astrometry
| radial_v = 70<ref name="SIMBAD" />
| prop_mo_ra = 137.993
| prop_mo_dec = -55.971
| pm_footnote = <ref name="GaiaDR3"/>
| parallax = 6.2327
| p_error = 0.1417
| parallax_footnote = <ref name="GaiaDR3"/>
}}

{{Starbox catalog
| names = [[Guide Star Catalog|GSC]] 6439.00120, X 03116-227<ref name=var />}}
{{Starbox reference
| Simbad = EF+Eri
}}
{{Starbox end}}

'''EF Eridani''' (abbreviated EF Eri, sometimes incorrectly referred to as EF Eridanus){{cn|date=July 2025}} is a [[variable star]] of the type known as [[Polar (cataclysmic variable)|polars]], [[AM Herculis]] stars, or [[magnetic]] [[cataclysmic variable]] stars. Historically it has varied between [[apparent magnitude]]s 14.5 and 17.3, although since 1995 it has generally remained at the lower limit. The star system consists of a [[white dwarf]] with a substellar-mass former star in [[orbit]].

==EF Eridani B==
The [[substellar object|substellar mass]] in orbit around the white dwarf is a star that lost all of its gas to the white dwarf. What remains is an object with a mass of 0.05 [[solar mass]]es ({{Solar mass|link=y}}), or about 53 [[Jupiter mass|Jupiter masses]] ({{Jupiter mass|link=yes}}), which is too small to continue [[nuclear fusion|fusion]], and does not have the composition of a super-[[Jupiter]], [[brown dwarf]], or white dwarf. In fact, this [[Compact star|stellar remnant]] doesn't fit into any commonly used category, and is as such essentially unique.<ref name=aaal354_L49/>

It is hypothesized 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 spiraled inward. Today, they are separated by a mere 700,000 km for an orbital period of {{val|81|u=min}}. The orbit is expected to continue to shrink due to [[gravitational radiation]].<ref name=aaal354_L49>{{cite journal | title=Evidence for a substellar secondary in the magnetic cataclysmic binary EF Eridani | author=Beuermann, K. | author2=Wheatley, P. | author3=Ramsay, G. | author4=Euchner, F. | author5=Gänsicke, B. T. | journal=Astronomy and Astrophysics Letters| volume=354 | pages=L49–L52 | date=February 2000 | bibcode=2000A&A...354L..49B|arxiv = astro-ph/0001183 }}</ref>

==See also==
* [[AM Herculis]]
* [[Cataclysmic variable star]]
* [[Polar (cataclysmic variable)]]
* [[Variable stars]]
* [[Compact star|Stellar remnants]]
* [[PSR J1719−1438 b|PSR J1719-1438 b]], a planetary-mass former star that was eroded by its binary star partner, [[PSR J1719-1438]]
*[[PSR J1544+4937 b]]
*[[Black Widow Pulsar|PSR B1957+20 b]]

==References==
{{reflist|refs=

<ref name="GaiaDR3">{{Cite Gaia DR3|5099482805904892288}}</ref>

<ref name="SIMBAD">{{cite web
| url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=EF+Eri
| title =V* EF Eri – Nova-like Star
| publisher=[[Centre de Données astronomiques de Strasbourg]]
| work=[[SIMBAD]]
| access-date=2009-12-17}}</ref>

<ref name="Williams">{{cite journal |last1=Williams |first1=G. |last2=Hiltner |first2=W. A. |title=Light curve of the optical counterpart of 2A 0311-227 |journal=Publications of the Astronomical Society of the Pacific |date=April 1980 |volume=92 |pages=178–182 |doi=10.1086/130643 |bibcode=1980PASP...92..178W |s2cid=120568192 |doi-access=free }}</ref>

<ref name=vincent2024>{{Cite journal |last1=Vincent |first1=O. |last2=Barstow |first2=M. A. |last3=Jordan |first3=S. |last4=Mander |first4=C. |last5=Bergeron |first5=P. |last6=Dufour |first6=P. |date=2024-01-26 |title=Classification and parameterization of a large <i>Gaia</i> sample of white dwarfs using XP spectra |bibcode=2024A&A...682A...5V |journal=Astronomy & Astrophysics |language=en |volume=682 |pages=A5 |doi=10.1051/0004-6361/202347694 |arxiv=2308.05572 |issn=0004-6361}}</ref>

}}

==External links==
* [https://web.archive.org/web/20041012120611/http://www.cnn.com/2004/TECH/space/10/05/space.star.reut/index.html (CNN) Faded star defies description]
* [https://web.archive.org/web/20030512095207/http://www.aavso.org/cgi-bin/searchcharts3.pl?name=EF+ERI AAVSO charts for EF Eridani]

{{Stars of Eridanus}}

{{DEFAULTSORT:EF Eridani}}
[[Category:Polars (cataclysmic variable stars)]]
[[Category:Eridanus (constellation)]]
[[Category:Objects with variable star designations|Eridani, EF]]
[[Category:Eclipsing binaries]]

EF Eridani - Revision history

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