85Peg A: which age for a low metallicity solar like star?

We explore the possible evolutionary status of the primary component of the binary 85 Pegasi, listed as a target for asteroseismic observations by the MOST satellite. In spite of the assessed `subdwarf' status, and of the accurate distance determination from the Hipparcos data, the uncertainties in the metallicity and age, coupled with the uncertainty in the theoretical models, lead to a range of predictions on the oscillation frequency spectrum. Nevertheless, the determination of the ratio between the small separation in frequency modes, and the large separation as suggested by Roxburgh (2004), provides a very good measure of the star age, quite independent of the metallicity in the assumed uncertainty range. In this range, the constraint on the dynamical mass and the further constraint provided by the assumption that the maximum age is 14 Gyr limit the mass of 85PegA to the range from 0.75 to 0.82Msun. This difference of a few hundreths of solar masses leads to well detectable differences both in the evolutionary stage (age) and in the asteroseismic properties. We show that the age determination which will be possible through the asteroseismic measurements for this star is independent either from the convection model adopted or from the microscopic metal diffusion. The latter conclusion is strengthened by the fact that, although metal diffusion is still described in an approximate way, recent observations suggest that the real stars suffer a smaller metal sedimentation with respect to the models.