AO Cassiopeiae (HD 1337A and B) is one of the most massive and luminous binary systems known. It is an elliptic and semidetached eclipsing binary consisting of two O stars (O9III) of 32 solar masses and 23 solar radii, and 30 solar masses and 15 solar radii, respectively. The two stars orbit so closely, with a period of only 3.52 days, that their surfaces are almost touching and highly distorted, and there is a continuous exchange of matter. The typical separation between the stars is somewhere between 2.5 and 3 times the primary stellar radius. AO Cas, also known as Pearce's Star, lies about 7,000 light-years away in the constellation Cassiopeia.