2002A&A...393L...7P

We report the detection of a broad, prominent 19.5µm dust emission feature in ISO-SWS spectra. It is especially conspicuous in the spectra of low mass-loss AGB stars belonging to the variability classes SRb and Lb with relatively high photospheric temperatures. The feature carrier is proposed to be Mg_0.1Fe_0.9O, a solid solution of MgO (periclase) and FeO (wustite). This dust species has cubic crystal symmetry like spinel, the carrier of the 13, 16.8 and 31.8µm spectral features, together with which the 19.5µm feature is being observed. A broad emission plateau between 11 and 15µm, which is attributable to amorphous Al₂O₃, is also detected together with the 19.5µm and the spinel features. As a consequence of our discovery, we postulate the existence of a distinct class of circumstellar shells, dominated by oxide and not by silicate dust.