AT Microscopii

Coordinates: 20^h 41^m 51.15925^s, −32° 26′ 06.8283″

AT Microscopii is a binary star system located at a distance of 35 ly (11 pc) from the Sun in the constellation of Microscopium. Both members are flare stars, meaning they are red dwarf stars that undergo random eruptions that increase their brightness. This pair lies physically near the red dwarf star AU Microscopii, which may mean they form a wide triple star system.

In 1926, Dutch-American astronomer Willem Jacob Luyten reported that the lines in the spectrum of this star underwent variation. A photographic plate taken in June 23, 1895 showed bright lines of hydrogen that were much weaker on a plate taken June 29, 1895. A photograph taken on July 1, 1903 showed no such lines. The net variation in brightness of the star was small, not exceeding 0.5 in magnitude. Luyten noted that the star has a large proper motion, changing its position by 0.43 seconds of arc between 1899 and 1923.

By 1927, the object was found to be a pair of stars with an angular separation of 2.95 arcseconds. Both were shown to be of 'dwarf Me' type, indicating they are red dwarfs with emission lines in their spectrum. This was the first such pair of matching dwarf Me stars to be discovered. The first parallax measurements of the pair showed an annual shift of about 0.1 arcseconds, while their radial velocity was measured to be +5 km/s away from the Sun. A nearby star HD 197981, later named AU Microscopii, was shown to have a similar radial velocity of +10 km/s. For this reason it was suggested that the three stars are physically associated.