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 ( T _ { eff } = 14000 \pm 1000 K ) primary and a G8 III type ( T _ { eff } \sim 5000 K ) secondary .
Absorption line positions of the secondary vary with a semi-amplitude of \sim 20 km s ^ { -1 } and a period of 27.5 days .
The system ’ s mass function is 2.3 \times 10 ^ { -2 } M _ { \odot } , and its orbital plane is \sim 13–15 \arcdeg tilted from the plane of the sky .
The primary ’ s v \sin i \sim 110 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-IR continuum suggest that the primary contributes \sim 60 % of the V –band flux , the disk contributes \sim 30 % , and the secondary \sim 10 % .
The system parameters , along with the interstellar extinction , suggest a distance of \sim 1 kpc , that the secondary does not fill its Roche lobe , and that the companions ’ mass ratio is q \sim 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 .