Absolute dimensions of eclipsing binaries. XXIII. The F-type system EI Cephei

We report new radial velocity measurements made with the Harvard-Smithsonia n Center for Astrophysics Digital Speedometers of the F-type double-lined e clipsing binary EI Cep (P = 8.4 days), in which the secondary is a metallic -line star. Using these and previous measurements from the literature, we r edetermine the masses of the components to a precision of 0.4%, 4 to 5 time s better than previously achieved. We also reanalyze UBV light curves for t he system from the literature with modern methods to determine the radii an d the effective temperatures. The resulting stellar parameters are M-A = 1. 7716 +/- 0.0066 M-., M-B = 1.6801 +/- 0.0062 M-., R-A = 2.896 +/- 0.048 R-. R-B = 2.329 +/- 0.044 R-., T-eff(A), = 6750 +/- 100 K, and T-eff(B), = 695 0 +/- 100 K. Further properties of the system are given in this paper: Both components are near the end oft heir main-sequence evolution, and a compar ison of the observational results with theoretical evolutionary models give s excellent agreement with models using two somewhat different treatments o f overshooting from the convective cores (but not with standard models). An age of 1.4 Gyr is derived for the roughly solar metallicity indicated by o ur low signal-to-noise ratio spectra. A precise spectroscopic determination of the metallicity of EI Cep would be very useful to place even tighter co nstraints on the models. The measured projected rotational velocities of th e stars (13 and 17 km s(-1) for EI Cep A and B, respectively) are very clos e to, but not quite synchronized with, the orbital motion. This is in agree ment with predictions from current tidal theories, which, however, also pre dict that orbital circularization has not yet occurred, although we detect no significant eccentricity.