Red giants with unusual dust-shells. Pt. 1

We describe a number of red giants which are classified as type M (i.e. oxygen-rich) but which are defined in the IRAS LRS Catalog as having SiC emission (a characteristic of carbon-rich environments). We show that the near-infrared spectra are characteristic of oxygen-rich stars, but that at least some appear to have carbon-to-oxygen abundance ratios close to unity (in other words they should be classified as S stars). We suggest that when C/O approaches unity, dust condensation theories assuming chemical and thermodynamic equilibrium may not be realistic. It is postulated that a number of carbon stars which have been found to have silicate dust features may also have C/O abundance ratios close to unity. These conclusions are all supported by near-infrared spectra which in each case confirm the optical classifications of the stars. (author)