SPECTRA OF R CRB DUST SHELLS

> discretionary time proposal as shown in the a&a iso special issue, the spectrum of r crb in the phts range (2.5um to 5um and 6um to 11.6um) is very unusual, in that it shows a broad plateau around 6um to 8um. although the dust shell is known to be very carbon rich, there was no sign of pah (polycyclic aromatic hydrocarbon) emission. r crb has been found to have no hydrogen in its stellar atmosphere. a second member of the class, ry sgr, shows a very similar spectrum, with a broad plateau and no pah emission, and again this star is known to have no hydrogen in its atmosphere. a second phts spectrum of r crb has shown the flux density of the plateau has changed over several months. we propose to observe two r crb stars, v854 cen and fg sge, which have a little hydrogen in their atmospheres (1) to see if they show pah emission (2) to see if they show this broad plateau and (3) to see if the spectrum changes over a period of months. v854 cen is nearing the end of its visibility to iso, so the request for time is urgent (its visibility ends on 6th september 1997). truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]