  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 +/- 0.018 Msun and M(B) = 0.6650 +/- 0.0079 Msun, with relative errors of
2% and 1%, respectively. We also determine the orbital parallax, pi(orb) =
46.08 +/- 0.27 mas, corresponding to a distance of 21.70 +/- 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 Teff(A) = 5200 +/- 100 K and Teff(B) =
4200 +/- 200 K. Together with detailed chemical abundance analyses from the
literature giving [Fe/H] approximately -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.
