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Wikipedia

40 Eridani (also known as Omicron² Eridani, or Keid, from the Arabic word qayd, (egg) shells) is a triple star system less than 16.5 light years away from Earth. It is in the constellation Eridanus. The primary star of the system, 40 Eridani A, is easily visible to the naked eye. The pair 40 Eridani B/C was discovered on January 31, 1783 by William Herschel.^{, p. 73} It was again observed by Friedrich Georg Wilhelm Struve in 1825 and by Otto Wilhelm von Struve in 1851. In 1910, it was discovered that although component B was a faint star, it was white in color. This meant that it had to be a small star; in fact it was a white dwarf, the first discovered.^{, p. 1}

General information

40 Eridani A is an red-orange main sequence dwarf of spectral type K1. Its two companion stars, 40 Eridani B and 40 Eridani C, are a 9th magnitude white dwarf (spectral type DA4) and an 11th magnitude red dwarf flare star (spectral type M4.5e) which has the variable star designation DY Eridani. Presumably, while B was a main sequence star, it was the most massive member of the system, but ejected most of its mass before it became a white dwarf. B and C orbit each other approximately 400 astronomical units from the primary star, A. Their orbit has a semimajor axis of 35 AU (which is also the approximate average distance between B and C) and is rather elliptical (eccentricity 0.410).

Potential for life

The habitable zone of 40 Eridani A, where a planet could exist with liquid water, is near 0.61 AU from A. At this distance a planet would complete a revolution in 203 Earth days, and 40 Eridani A would appear about 30% wider than Sol does on Earth. An observer on a planet in the 40 Eridani A system would see the B/C pair as unusually bright (magnitudes -8 and -6) white and red stars in the night sky. This is not bright enough to diminish the darkness at night, though they would be visible in daylight (assuming an Earth-normal atmosphere). (By comparison, Earth's full moon is magnitude -12.6, and Venus at its brightest is -4.7.) It is extremely unlikely that habitable planets exist around the B star because planets circling 40 Eridani B would likely have been destroyed or sterilized by its evolution into a white dwarf. As for 40 Eridani C, it is prone to flares, which cause large momentary increases in the emission of X-rays as well as visible light. This would be lethal to life on a planet in the habitable zone.