The unusual carbon-rich dust around the R CrB variables

When the dust shell around R CrB was observed with the low resolution spectrometer on ISO (ISOPHOT-S), from 2.5 to 5 mu m and 5.8 to 11.6 mu m, an unusual broad plateau of emission between 5 and 8 mu m was found. Three further observations of R CrB were taken during the ISO mission, showing that the broad emission plateau varies in strength and shape. A second member of the R CrB class of stars, RY Sgr, was observed three times! and these spectra showed that RY Sgr has the same type of broad, variable plateau. A third member of the class, V854 Cen, has detectable hydrogen in its stellar atmosphere (R CrB and RY Sgr have very little hydrogen), and the TSOPHOT-S spectrum of the dust around this star showed PAH emission features as well as the plateau emission.