AU Pegasi - The binary Cepheid outside the instability strip

Continued spectroscopy of AU Peg, a peculiar Type II Cepheid, has allowed determination of the orbital period, separation of the pulsation and orbital velocities, and solution of the single-line binary orbit. The companion is probably more massive than AU Peg, and is probably a compact object. It is sufficiently close that mass transfer must have occurred within the system in the past. A high dispersion abundance analysis shows that AU Peg has Fe/H = +0.1, and is considerably cooler than normal for its period. AU Peg is outside the normal instability strip, lying approximately 500 K cooler than the normal red edge. One possible explanation, a high helium abundance, is discussed. The large period increase remains to be explained. Interactions with binary companions may be important for other Type II Cepheids.