Delta Scuti stars in the HR diagram

A sample of 75 delta Scuti stars with parallaxes known to better than 15 per cent is examined. A significant fraction of them (similar to 10 per cent) lie beyond the core-hydrogen exhaustion phase in the HR diagram, compared with the models of Schaller et al. (1992) which have a moderate overshooting distance d(over)/H-p = 0.2. The Period-Luminosity-Colour relation based on these new data shows as much scatter as previous ones, confirming that the scatter is essentially due to the variety of the excited modes, rather than to uncertainties in the absolute magnitude. The mass, interpolated in recent evolutionary tracks (assuming standard evolution), is given for each star, as well as the pulsation constant Q corresponding to the main period. An attempt to determine the pulsation mode using a formula by Fitch (1981) remained inconclusive. The observed parallaxes of SX Phe and AI Vel correspond very well to those predicted on theoretical grounds by Petersen & Christensen-Dalsgaard (1996).