Absolute Dimensions of Eclipsing Binaries. XXIII. The F-Type System EI Cephei

We report new radial velocity measurements made with the Harvard-Smithsonian Center for Astrophysics Digital Speedometers of the F-type double-lined eclipsing 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 redetermine the masses of the components to a precision of 0.4%, 4 to 5 times better than previously achieved. We also reanalyze UBV light curves for the system from the literature with modern methods to determine the radii and the effective temperatures. The resulting stellar parameters are M_A=1.7716+/-0.0066 M_solar, M_B=1.6801+/-0.0062 M_solar, R_A=2.896+/-0.048 R_solar, R_B=2.329+/-0.044 R_solar, T^A_eff=6750+/-100 K, and T^B_eff=6950+/-100 K. Further properties of the system are given in this paper. Both components are near the end of their main-sequence evolution, and a comparison of the observational results with theoretical evolutionary models gives excellent agreement with models using two somewhat different treatments of 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 our low signal-to-noise ratio spectra. A precise spectroscopic determination of the metallicity of EI Cep would be very useful to place even tighter constraints on the models. The measured projected rotational velocities of the stars (13 and 17 km s^-1 for EI Cep A and B, respectively) are very close to, but not quite synchronized with, the orbital motion. This is in agreement with predictions from current tidal theories, which, however, also predict that orbital circularization has not yet occurred, although we detect no significant eccentricity. Some of the observations reported in this paper were obtained at the Multiple Mirror Telescope Observatory, a facility operated jointly by the University of Arizona and the Smithsonian Institution.