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We present a detailed study of the F-type detached eclipsing binary BK Peg,based on new photometric and spectroscopic observations. The two components,which have evolved to the upper half of the main-sequence band, are quitedifferent with masses and radii of (1.414 +/- 0.007 Msun, 1.988 +/- 0.008 Rsun)and (1.257 +/- 0.005 Msun, 1.474 +/- 0.017 Rsun), respectively. The 5.49 dayperiod orbit of BK Peg is slightly eccentric (e = 0.053). The measuredrotational velocities are 16.6 +/- 0.2 (primary) and 13.4 +/- 0.2 (secondary)km/s. For the secondary component this corresponds to (pseudo)synchronousrotation, whereas the primary component seems to rotate at a slightly lowerrate. We derive an iron abundance of [Fe/H] =-0.12 +/- 0.07 and similarabundances for Si, Ca, Sc, Ti, Cr and Ni. Yonsei-Yale and Victoria-Reginaevolutionary models for the observed metal abundance reproduce BK Peg at agesof 2.75 and 2.50 Gyr, respectively, but tend to predict a lower age for themore massive primary component than for the secondary. We find the same agetrend for three other upper main-sequence systems in a sample of well studiedeclipsing binaries with components in the 1.15-1.70 Msun range, whereconvective core overshoot is gradually ramped up in the models. We also findthat the Yonsei-Yale models systematically predict higher ages than theVictoria-Regina models. The sample includes BW Aqr, and as a supplement we havedetermined a [Fe/H] abundance of -0.07 +/- 0.11 for this late F-type binary. Wepropose to use BK Peg, BW Aqr, and other well-studied 1.15-1.70 Msun eclipsingbinaries to fine-tune convective core overshoot, diffusion, and possibly otheringredients of modern theoretical evolutionary models.

Absolute dimensions of eclipsing binaries. XXVIII. BK Pegasi and other F-type binaries: Prospects for calibration of convective core overshoot