Consequences of Hipparcos parallaxes for stellar evolutionary models - Three Hyades binaries: V 818 Tauri, 51 Tauri, and theta(2) Tauri

Three binary systems in the Hyades open cluster (51 Tau, V818 Tau, and thet a(2) Tau), with known metallicity and good Johnson photometric data are use d to test the validity of three: independent sets of stellar evolutionary t racks. A statistical method is described and applied to the colour-magnitud e diagram of the six selected components, giving rise to chi(2)-contours in the metallicity-age plane. The effects of the Hipparcos parallaxes on thes e confidence regions are studied in detail for these binaries through a com parison with very accurate but older orbital parallaxes. There are no signi ficant differences in 51 Tau and theta(2) Tau, and hence the test validates the sets of theoretical tracks used. However the orbital parallax of V 818 Tau leads to metallicities too large in comparison with the observed range , while the Hipparcos parallax give consistent results, at the 1 sigma leve l. Comparisons between the predicted and measured masses are also made acid show a good agreement. However, even if the Padova tracks predict correct masses for each component of V 818 Tau, there is no Padova track that fits both components simultaneously in the mass-radius diagram. Finally, since t he individual masses of these binaries are accurately known, some issues ab out the influence of the Hipparcos parallaxes on their location in the mass -luminosity diagram are also discussed.