ABSOLUTE DIMENSIONS OF ECLIPSING BINARIES .19. BW AQUARII - A LATE F-TYPE INDICATOR OF OVERSHOOTING

Accurate absolute dimensions (+/- 2%) of the components of the southern detached late F-type eclipsing binary BW Aqr have been determined from an analysis of new complete uvby light curves (Gronbech et al. 1987) combined with results from a recent radial velocity study (Imbert 1987). Both components have evolved to the upper main-sequence, and the more massive one (M = 1.49 M., R = 2.05 R.) has become about 100 K cooler than the other, for which M = 1.39 M. and R = 1.78 R.. Only few systems of this kind are known in the theoretically not yet well studied transition region where stars develop convective cores. Standard models of solar composition fit the less massive BW Aqr A well, whereas this is not the case for BW Aqr B. Models including moderate overshooting provide a more satisfactory fit to both components for a common age (almost-equal-to 2.5 Gyr). The older, slightly less massive but otherwise similar system BK Peg also indicates the need of overshooting. The metal abundance of both systems should be measured for a definitive confrontation. The orbit of BW Aqr is eccentric (e = 0.17) and the rotations of both stars are clearly below the pseudosynchronous and also below the synchronous values. Slow apsidal motion (U = 7400 yr) is present with about one third of the observed rate due to relativistic effects. The stars are found to be slightly less centrally condensed than predicted by all evolutionary models. More accurate times of minima should be obtained through the next decades for a deeper study of the apsidal motion.