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Description
We report the results of a long-term spectroscopic monitoring of the FS CMa type object MWC 728. We found that it is a binary system with a B5Ve (Teff=14000+/-1000K) primary and a G8III type (Teff~5000K) secondary. Absorption line positions of the secondary vary, with a semi-amplitude of ~20km/s and a period of 27.5 days. The system's mass function is 2.3x10^-2^M_{sun}_, and its orbital plane is ~13{deg}-15{deg} tilted from the plane of the sky. The primary's vsini~110km/s, combined with this tilt, implies that it rotates at a nearly breakup velocity. We detected strong variations of the Balmer and HeI emission-line profiles on timescales from days to years. This points to a variable stellar wind of the primary in addition to the presence of a circum-primary gaseous disk. The strength of the absorption-line spectrum, along with the optical and near infrared (IR) continuum, suggest that the primary contributes ~60% of the V-band flux, the disk contributes ~30%, and the secondary contributes ~10%. The system parameters, along with the interstellar extinction, suggest a distance of ~1kpc, that the secondary does not fill its Roche lobe, and that the companions' mass ratio is q~0.5. Overall, the observed spectral variability and the presence of a strong IR-excess are in agreement with a model of a close binary system that has undergone a non-conservative mass-transfer.
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