Four-colour photometry of eclipsing binaries, XA: photometric elements, absolute dimensions and helium abundance of chi 2 Hydrae.

Summary. The light curves of the bright double-lined B-type eclipsing binary x2 Hydrae have been observed in the Str5mgren four-colour uvby system. The model of an eclipsing binary and the light curve simulation programme WINK described by Wood (1971, 1972) have been used to determine the photometric elements listed in Table 4. Combination of these with the spectroscopic data by Andersen (1975) yields the absolute dimensions given in Table 7. Comparison with theoretical stellar models (Fig. 3) by Hejlesen in his unpublished extension of Hejlesen et al. (1972) shows much better agreement than obtained with the photometric elements previously determined by Mauder (1970): Firstly, the same age (1.8 x 108 years) is found both for the considerably evolved primary and for the essentially unevolved secondary component. Secondly, one single value of the helium abundance (Y= 0.28+0.04) is obtained for both components. Key words: eclipsing binary - photometric elements - absolute stellar dimensions - He content