Absolute properties of the eclipsing binary star FS monocerotis

We present photometric observations in B and V as well as spectroscopic obs ervations of the 1.9 day-period, double-lined main-sequence eclipsing binar y FS Mon. From the analysis of the light curves and radial velocity curves, we have determined the masses and radii of the components: 1.631 +/- 0.012 M. and 2.051 +/- 0.012 R. for the primary component; 1.461 +/- 0.010 M. an d 1.629 +/- 0.012 R. for the secondary component. The formal uncertainties are all less than 1%. Based on our light curves as well as on the combined- light photometry of the system, we estimate effective temperatures of 6715 +/- 100 K for the primary and 6550 +/- 100 K for the secondary component, c orresponding to spectral types of F2 and F4, respectively. Projected rotati onal velocities (upsilon sin i) from the spectograms are measured as 52 +/- 2 km s(-1) and 43 +/- 3 km s(-1), respectively, from the spectrograms and are consistent with rotation that is synchronous with the orbital motion. E volutionary tracks from current models are in good agreement with the obser vations for a system age of about 1.6 x 10(9) yr with slightly nonsolar che mical composition. We find the best match for models with a slightly lower helium abundance and a higher metal abundance than the Sun. An accurate met allicity determination for FS Mon is needed to constrain the models further .