ABSOLUTE DIMENSIONS OF ECLIPSING BINARIES .22. THE UNEVOLVED F-TYPE SYSTEM HS HYDRAE

We have redetermined the masses of the unevolved F-type eclipsing bina ry HS Hya (P=1.57 days) from 70 observations with the CfA digital spee dometers, combined with other observations from the literature. Analys is of our radial velocities determined with a two-dimensional cross-co rrelation technique shows the presence of a third star in the system, which we find to be most likely an M0 dwarf with a mass of similar to 0.5 M. in a 190-day orbit of small eccentricity and apparently co-plan ar with the eclipsing system. We also reanalyze an earlier set of ligh t curves in the uvby system, using modern analysis techniques and comb ining all available spectroscopic and photometric data in the analysis . The resulting masses for the eclipsing stars are M-A=1.255+/-0.008 M . and M-B=1.219+/-0.007 M., similar to 6% smaller and similar to 6 tim es more accurate than previous values. The radii are R-A=1.275+/-0.007 R. and R-B=1.216+/-0.007 R., and the effective temperatures T-A=6500/-50 K, T-B=6400+/-50 K; further properties of the stars are given in Table 7. Current stellar evolution models for the observed metallicity (slightly below solar) match the observed properties of both stars in HS Hya very well for a single, but strongly model-dependent age of si milar to 1.0 Gyr. A precise spectroscopic determination of [Fe/H] for HS Hya is needed to further refine comparison with theoretical models. (C) 1997 The American Astronomical Society. [S0004-6256(97)04412-9].