AO Cassiopeiae

AO Cassiopeiae, also known as Pearce's Star, is a binary system composed of an O8 main sequence star and an O9.2 bright giant that respectively weigh anywhere between 20.30 and 57.75 times and 14.8 and 31.73 times the mass of the Sun.

The AO Cas system is an eclipsing binary with a period of roughly 3.5 days, with the apparent magnitude ranging between 6.07 and 6.24. Stars of this brightness are generally just visible to the unaided eye in dark skies in semirural locations. The component stars are so close to each other they are ellipsoidal (egg-shaped). AO Cas is considered a contact binary, with both stars at or near their Roche lobes.