First determination of the distance and fundamental properties of an eclipsing binary in the andromeda galaxy

We present the first detailed spectroscopic and photometric analysis of an eclipsing binary in the Andromeda Galaxy (M31). This is a 19.3 mag semidetached system with late O and early B spectral type components. From the light and radial velocity curves we have carried out an accurate determination of the masses and radii of the components. Their effective temperatures have been estimated by modeling the absorption- line spectra. The analysis yields an essentially complete picture of the properties of the system, and hence an accurate distance determination to M31. The result is d = 772 +/- 44 kpc [(m - M)(0) = 24.44 +/- 0.12 mag] . The study of additional systems, currently in progress, should reduce the uncertainty of the M31 distance to better than 5%..