The spectra of the IRAS low-resolution-spectrometer in tape form have been submitted to a systematic morphological analysis, using classical quantitative discriminants (O.Gal et al. 1987, A & A 183, 29; Y. Baron et al. 1987, A & A 186, 271; R. Papoular 1988, A & A , in press). Spectra which display the 11.5μ feature of SiC fall into 4 classes of average spectral excesses. They differ by the width of the SiC feature and by the presence or absence of secondary features at ~8.6, ~11.7 and ~12.8μm.

A majority of these spectra have a lower 12-25μm colour temperature than do most optically selected C-stars, presumably because of thicker dust envelopes. While most spectra belong to LRS class 4n, 20% of the total were found among the brightest 20% of the much larger class In, suggesting that the relative abundance of C-stars is much higher than previously assumed.