The connection between sequence D and sequence E red giant variables and asymmetric PNe

Red giant stars showing Long Secondary Periods, known as sequence D variables, make up about 30% of all luminous AGB stars. This is similar to the fraction of planetary nebulae that are known to be asymmetric. One of the models for sequence D variables is that they are binaries, a favourite explanation for asymmetric planetary nebulae. However, recent observations of the sequence D stars rule out binarity as an explanation for their behaviour. If there is a connection between sequence D variables and asymmetric planetary nebulae, it is not due to binarity. Sequence E variables are ellipsoidal red giant binaries, in which a red giant is distorted by its unseen orbiting companion. We have used the observed frequency of sequence E binaries relative to non-variable red giants to estimate the fraction of low mass stars in the LMC that terminate their red giant evolution by a common envelope event, most likely producing a strongly asymmetric planetary nebula.