Red giants with unusual dust shells - I. The database.

A number of red giants are classified as type M (oxygen-rich) but are defined in the IRAS LRS Catalog as having SiC emission (a characteristic of carbon-rich environments). It is shown that the NIR spectra are characteristic of oxygen-rich stars, but that at least some appear to have C/O abundance ratios close to unity (in other words they should be classified as S stars). It is suggested 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.