Theoretical isochrones and main sequences for old disk population stars.

Theoretical isochrones have been constructed for fourteen mixtures in the ranges of chemical compositions (0.05 < Y< 0.45) and (0.01 <z <0.06) and for ages relevant to the old disk population stars. The effects of autoionization absorption as computed by Watson were added to the Cox-Stewart radiative opacities. Comparison with the colour-magnitude diagram of M67 and NGC 188 does not favour supermetallicity, but rather normal abundances. Helium abundance by mass Yin the range of 0.25-0.35 is found. The distance of the Hyades cluster is discussed in terms of main sequence models extended to higher masses. The distance modulus is consistent with the models for a shift of 0.27 mag. from that given by the convergent point method. This shift would be smaller for lower interior opacities (i.e. if we have overestimated the heavy element abundance Z, or alternately if Watson's autoionization opacities are too large). Key words: stellar evolution - disk population - Hyades