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Among its great findings, the IRAS mission showed the existence of anunidentified mid-IR feature around 21 um. Since its discovery, this feature hasbeen detected in all C-rich proto-PNe of intermediate spectral type (A-G) and -weakly - in a few PNe and AGB stars, but the nature of its carriers remainsunknown. In this paper, we show the detection of this feature in the spectra ofthree new stars transiting from the AGB to the PN stage obtained with theSpitzer Space Telescope. Following a recent suggestion, we try to model theSEDs of our targets with amorphous carbon and FeO, which might be responsiblefor the unidentified feature. The fit thus obtained is not completelysatisfactory, since the shape of the feature is not well matched. In theattempt to relate the unidentified feature to other dust features, we retrievedmid-IR spectra of all the 21-um sources currently known from ISO and Spitzeron-line archives and noticed a correlation between the flux emitted in the21-um feature and that emitted at 7 and 11 um (PAH bands and HAC broademission). Such a correlation may point to a common nature of the carriers.

Identification of three new proto-Planetary Nebulae exhibiting the unidentified feature at 21 um