Luyten 726-8 AB is a binary star system in the southwestern part of the constellation Cetus, the Whale. Located only 8.57 light-years away, it is the closest star system in the constellation Cetus, and the sixth closest system to our own.
With visual magnitudes of 15.3 and 15.8, this pair of red dwarf stars is too faint to be seen with the naked eye. It was discovered in 1949 by Willem Jacob Luyten; he noted its exceptionally high proper motion (3.37" per year) while compiling a catalogue of high proper motion stars.
Components
The components of Luyten 726-8 AB have a combined mass of about 20% of the Sun's, but less than 1/9000th of its luminosity. They are separated on average by about 5.5 AU, in a highly elliptical orbit of eccentricity 0.62 that carries them between 2.1 and 8.8 AU apart over an orbital period of 26.5 years.
Luyten 726-8 A is a cool, dim, main-sequence red dwarf of spectral class M5.6 Ve. It has 11% of the Sun's mass, 14% of its diameter, and less than 1/16,000th of its luminosity. Luyten 726-8 B, even dimmer, is a main-sequence class M6.0 Ve red dwarf with only 10% of the Sun's mass, 14% of its diameter, and less than 1/25,000th of its luminosity.
Variability
Both components are flare stars, and the fainter member of the system is an extreme example. It has the variable star designation UV Ceti. It can increase in brightness by a factor of five in less than a minute, then fall back to its normal luminosity within two or three minutes, then flare again suddenly after several hours. In 1952, UV Ceti was observed flaring to 75 times its normal brightness in only 20 seconds.
Although UV Ceti was not the first flare star discovered, it is the most prominent example of such a star, and in fact flare stars are sometimes called "UV Ceti" variables. Luyten 726-8 A is also as a flare star, designated BL Ceti, although less dramatic in its brightness variations than its companion.