2007A&A...469..285V

Precise stellar masses and radii provide unique information on stellar evolution. In a Galactic context, they may also provide information on the evolution of the Solar neighbourhood. We aim to determine absolute dimensions for the mid B-type eclipsing binary U Ophiuchi and compare the inferred ages and chemical compositions to those of other binary stars with masses near 5M_☉. We determine masses, radii, logg, logT_eff, and luminosities for the stars in U Oph from new radial velocities and uvby light curves. By improving the Wilson-Devinney code, we also derive precise apsidal-motion and light-time orbits of this triple system, using 353 times of minimum over 120 years. Finally, we compare the data for U Oph and three similar systems with the predictions of stellar models. The stars in U Oph have masses of 5.27±0.09M_☉ and 4.74±0.07M_☉, radii of 3.48±0.02R_☉ and 3.11 ±0.03R_☉; we argue that systematic errors are negligible. The apsidal motion period is exceptionally short, U∼21yr, while the light-time orbit has P₃∼38.4yr. The precise logg values for DI Her, U Oph, V760 Sco, and MU Cas, all within 10% of 5M_☉, place them successively from the ZAMS to the TAMS, at ages of 5-100x10⁶yr. Current stellar evolution models fit both stars in each system very well if the (otherwise unconstrained) metal abundance Z is adjusted in each case. More accurate data and/or binary systems with larger mass ratios are needed to actually test stellar models near 5M_☉. The different Z values found for these young, nearby systems suggest that disk stars form with a range of metal abundances even today, but more data are needed to confirm this result.