Abstract

Variable red giants occupy several sequences in the period—luminosity plane. In this talk I will focus on sequence E. Sequence E stars are distorted red giant binaries, known as ellipsoidal variables, lying mostly on the RGB . Their ellipsoidal nature was hypothesised from the light curve shape only; we present velocity curves that confirm this theory. We also use the observed fraction of red giants which are sequence E stars to predict the fraction of PN that have close binary post-common envelope central stars. Approximately ten percent of sequence E stars are in eccentric orbits, although binary evolution theory predicts their orbits should have been quickly circularised by tidal forces once becoming red giants. We present preliminary results in our search for the mechanism that maintains eccentricity in these binaries.