  Using the technique of spectral disentangling, it is possible to determine
the individual spectra of the components of a multiple star system from
composite spectra observed at a range of orbital phases. This method has
several advantages: it is unaffected by line blending, does not use template
spectra, and returns individual component spectra with very high
signal-to-noise ratios. The disentangled spectra of a binary star system are
very well suited to spectroscopic analysis but for one problem: the absolute
spectral line depths are unknown because this information is not contained in
the original spectra (unless there is one taken in eclipse) without making
assumptions about the spectral characteristics of the component stars. Here we
present a method for obtaining the atmospheric parameters of the component
stars by the constrained fitting of synthetic spectra to observed and
disentangled spectra. Disentangled spectra are fitted using synthetic spectra
and a genetic algorithm in order to determine the effective temperatures,
surface gravities and relative light contributions of the two stars in a binary
system. The method is demonstrated on synthetic spectra and then applied to the
eclipsing binary V615 Per, a member of the young open cluster NGC 869 (h
Persei). The method works well for disentangled spectra with signal-to-noise
ratios of 100 or more. For V615 Per we find a normal He abundance but an Mg
abundance, which indicates bulk metallicity, a factor of two lower than typical
for nearby OB stars.
