Abstract
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 B5 v e ( = 14,000 ± 1000 K) primary and a G8 iii type ( 5000 K) secondary. Absorption line positions of the secondary vary, with a semi-amplitude of ∼20 km s −1 and a period of 27.5 days. The system's mass function is 2.3 × 10 −2 , and its orbital plane is ∼13°–15° tilted from the plane of the sky. The primary's km s −1 , combined with this tilt, implies that it rotates at a nearly breakup velocity. We detected strong variations of the Balmer and He i 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 ∼1 kpc, that the secondary does not fill its Roche lobe, and that the companions' mass ratio is 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.