Absolute Properties of the Eclipsing Binary Star FS Monocerotis

We present photometric observations in B and V as well as spectroscopic observations of the 1.9 day–period, double-lined main-sequence eclipsing binary 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 ⊙ and 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, corresponding to spectral types of F2 and F4, respectively. Projected rotational velocities ( v 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. Evolutionary tracks from current models are in good agreement with the observations for a system age of about 1.6 × 10 9 yr with slightly nonsolar chemical composition. We find the best match for models with a slightly lower helium abundance and a higher metal abundance than the Sun. An accurate metallicity determination for FS Mon is needed to constrain the models further.