  We present the first detailed spectroscopic and photometric analysis of an
eclipsing binary in the Andromeda Galaxy (M31). This is a 19.3-mag
semi-detached system with components of late-O and early-B spectral types. 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 from the modeling of 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 in the M31 distance to
better than 5%.
