 In 2005 Ignasi Ribas (CSIC, Institute for Space Studies of Catalonia ( IEEC) ) and colleagues announced the discovery of an eclipsing binary star in the Andromeda Galaxy. The binary star, designated M31VJ00443799 + 4129236, has two luminous and hot blue stars of types O and B. By studying the eclipses of the stars, which occur every 3.54969 days, astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars, they were able to measure their absolute magnitude. When the visual and absolute magnitudes are known, the distance to the star can be measured. The stars lie at a distance of 2.52 × 10 ^ 6 ± 0.14 × 10 ^ 6 ly (1.594 × 1011 ± 8.9 × 109 AU) and the whole Andromeda Galaxy at about 2.5 × 10 ^ 6 ly (1.6 × 1011 AU). This new value is in excellent agreement with the previous, independent Cepheid-based distance value.
