EF Eridani

EF Eridani (abbreviated EF Eri, sometimes incorrectly referred to as EF Eridanus)^{[citation needed]} 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 with a substellar-mass former star in orbit.

EF Eridani

Visual band light curves for EF Eridani, adapted from Williams and Hiltner (1980)[1]
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Eridanus
Right ascension	03h 14m 13.25262s[2]
Declination	−22° 35′ 42.9001″[2]
Apparent magnitude (V)	14.5–17.3[3]
Characteristics
Spectral type	DA[4]
Variable type	E + XM[3]
Astrometry
Radial velocity (Rv)	70[5] km/s
Proper motion (μ)	RA: 137.993 mas/yr[2] Dec.: -55.971 mas/yr[2]
Parallax (π)	6.2327±0.1417 mas[2]
Distance	520 ± 10 ly (160 ± 4 pc)
Other designations
GSC 6439.00120, X 03116-227[3]
Database references
SIMBAD	data

EF Eridani B

The 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 masses (M_☉), or about 53 Jupiter masses (M_J), which is too small to continue fusion, and does not have the composition of a super-Jupiter, brown dwarf, or white dwarf. In fact, this stellar remnant doesn't fit into any commonly used category, and is as such essentially unique.^[6]

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 81 min. The orbit is expected to continue to shrink due to gravitational radiation.^[6]

See also

• AM Herculis
• Cataclysmic variable star
• Polar (cataclysmic variable)
• Variable stars
• Stellar remnants
• PSR J1719-1438 b, a planetary-mass former star that was eroded by its binary star partner, PSR J1719-1438
• PSR J1544+4937 b
• PSR B1957+20 b