We report spectroscopic and interferometric observations of the moderately metal-poor double-lined binary system HD 195987 , with an orbital period of 57.3 days .
By combining our radial-velocity and visibility measurements we determine the orbital elements and derive absolute masses for the components of M _ { A } = 0.844 \pm 0.018 M _ { \sun } and M _ { B } = 0.6650 \pm 0.0079 M _ { \sun } , with relative errors of 2 % and 1 % , respectively .
We also determine the orbital parallax , \pi _ { orb } = 46.08 \pm 0.27 mas , corresponding to a distance of 21.70 \pm 0.13 pc .
The parallax and the measured brightness difference between the stars in V , H , and K yield the component absolute magnitudes in those bands .
We also estimate the effective temperatures of the stars as T _ { eff } ^ { A } = 5200 \pm 100 K and T _ { eff } ^ { B } = 4200 \pm 200 K. Together with detailed chemical abundance analyses from the literature giving [ Fe/H ] \approx - 0.5 ( corrected for binarity ) and [ \alpha /Fe ] = +0.36 , we use these physical properties to test current models of stellar evolution for metal-poor stars .
Among the four that we considered , we find that no single model fits all observed properties at the measured composition , although we identify the assumptions in each one that account for the discrepancy and we conclude that a model with the proper combination of assumptions should be able to reproduce all the radiative properties .
The indications from the isochrone fits and the pattern of enhancement of the metals in HD 195987 are consistent with this being a thick disk object , with an age of 10–12 Gyr .