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Carbon stars

True `carbon stars' are very cool supergiants with carbon-rich spectra. They are not thought to be particularly hydrogen-deficient, but are low-mass stars which have almost completed their nuclear burning stages. They will soon contract to become a white dwarf, probably illuminating a planetary nebula first. They may provide an important clue to understanding the RCB phenomenon.

Weak RCB-like fading events have recently been discovered in carbon stars ([Whitelock 1997]), suggesting that the process of carbon condensation is not confined to classical RCB stars. These very large stars also show pulsation-like behaviour with chaotic periods. Recent theoretical work and new observations ([Löbel et al. 2000]) show that although these pulsations may be `radial', they may not be spherical. Because of the enormous size of these stars, the pulsations may not even be coherent over the whole stellar surface. Thus, as the surface is pushed outwards more over one part of the star than another, that part becomes cooler and less dense. Shocks are more likely to develop and trigger dust condensation over small areas of the photosphere than over the whole surface. In carbon stars, these events may be very localized, obscuring only a fraction of the stellar surface. RCB stars, on the other hand, are not so large as the carbon stars ( $\sim 50$     $\rm R_{\odot}$ compared with $\sim 1000$     $\rm R_{\odot}$). As a `puff' is ejected from the star and expands outwards, it is more likely to obscure the whole star while it is still opaque than it would if it were a carbon star.

Detailed numerical modelling of RCB pulsations will be needed to determine whether their pulsations could be aspherical. It is conceivable that careful polarimetric observations over several pulsation cycles might also detect such asymmetries.