Omicron Andromedae

Omicron Andromedae (ο And, ο Andromedae) is a star system in the constellation Andromeda. It is approximately 692 light years from Earth. The system as a whole is classified as a blue-white B-type giant, with a mean combined apparent magnitude of +3.62.

Omicron Andromedae is a multiple star containing at least four components. It is thought to consist of two close pairs in a wider orbit, making a four-star system. This star system has a peculiar velocity of 34.5 ± 5.9 km/s.

The components A and B were first resolved in 1949, when they were reported to be separated by less than 0.1". In 1975 they were separated by 0.375" and by 2014 by only 0.21". An orbit has been derived with a period of 117 years.

In 1975, a companion was discovered by speckle interferometry only 0.05" from component A. Components Aa and Ab orbit every 5.6 years.

A fourth component in the system was suspected and in 1988 it was confirmed. Although a clear 33.01 day period was seen, it was unclear which component the fourth star was in orbit with. Eventually, it was settled that component B was a close spectroscopic binary including the fourth star.

Omicron Andromedae is a Gamma Cassiopeiae type variable star and the system's brightness varies from magnitude +3.58 to +3.78. The variable component is the brightest and most massive star in the system, Aa.

The spectrum is predominantly that of a B6 giant star, from the brightest component in the system. It is a shell star and the spectrum contains emission lines with variable profiles.

Spectral lines similar to an A2 star are also detectable in the spectrum and these are thought to originate in the B component.