An infrared sequence in the late stages of stellar evolution

Late-type stars showing the 9.7-μm silicate feature in emission or absorption are found to occupy different parts of the colour-colour diagram. Furthermore, these objects are separated in colour from planetary nebulae which have high radio surface brightness. The colour distribution of these classes of objects suggests the presence of a continuous infrared sequence, connecting the asymptotic giant branch and planetary nebulae phases, which allows for the search for transitional objects between these two evolutionary phases. (author)