Features of amorphous silicates around 10 and 18 m m are found in most of the spectra. The 13 m m feature normally attributed to corundum is very prominent in most of the semiregulars of our sample. They are only prominent in spectra showing a strong 13 m m feature. The most plausible interpretation of these lines is an atomic origin and formation under conditions which also favour 13 m m dust. It is also possible that there is a direct physical connection between these emitters and the 13 m m dust.
All figures are available with two different resolutions.