  We derive the theoretical red edge of the pulsating GW Vir stars by using
full evolutionary calculations that involve mass loss and diffusion. We put the
emphasis on the fact that the specific mass loss law used in the evolutionary
computations determines the red edge's position. By combining this specific
property with the observed location of the red edge in the effective
temperature-surface gravity domain, we obtain interesting constraints on
possible mass loss laws for PG 1159 stars.
