Gravity-mode Instabilities in Models of Post–Extreme Horizontal Branch Stars: Another Class of Pulsating Stars?

We present new results of a stability analysis of realistic models of post-extreme horizontal branch stars. We find that g -mode instabilities develop in some of these models as a result of a potent -mechanism associated with the presence of an active H-burning shell. The -process drives low-order and low-degree g modes with typical periods in the range 40-125 s. The unstable models populate a broad instability strip covering the interval 76,000 K ≳ T eff ≳ 44,000 K and are identified with low-mass DAO white dwarfs. They descend from stars that reach the zero-age "extended" horizontal branch with H-rich envelope masses M env ≳0.0010 M ☉ . Our computations indicate that some DAO stars should show luminosity variations resulting from pulsational instabilities. We suggest looking for brightness variations in six particularly promising candidates.