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Mid-IR (8 - 13 micron) interferometric data of four oxygen-rich AGB stars (RAql, R Aqr, R Hya, and W Hya) and one carbon-rich AGB star (V Hya) wereobtained with MIDI/VLTI between April 2007 and September 2009. The spectrallydispersed visibility data are analyzed by fitting a circular fullylimb-darkened disk (FDD). Results. The FDD diameter as function of wavelengthis similar for all oxygen-rich stars. The apparent size is almost constantbetween 8 and 10 micron and gradually increases at wavelengths longer than 10micron. The apparent FDD diameter in the carbon-rich star V Hya essentiallydecreases from 8 to 12 micron. The FDD diameters are about 2.2 times largerthan the photospheric diameters estimated from K-band observations found in theliterature. The silicate dust shells of R Aql, R Hya and W Hya are locatedfairly far away from the star, while the silicate dust shell of R Aqr and theamorphous carbon (AMC) and SiC dust shell of V Hya are found to be closer tothe star at around 8 photospheric radii. Phase-to-phase variations of thediameters of the oxygen-rich stars could be measured and are on the order of15% but with large uncertainties. From a comparison of the diameter trend withthe trends in RR Sco and S Ori it can be concluded that in oxygen-rich starsthe overall larger diameter originates from a warm molecular layer of H2O, andthe gradual increase longward of 10 micron can be most likely attributed to thecontribution of a close Al2O3 dust shell. The chromatic trend of the GaussianFWHM in V Hya can be explained with the presence of AMC and SiC dust. Theobservations suggest that the formation of amorphous Al2O3 in oxygen- richstars occurs mainly around or after visual minimum. However, no firmconclusions can be drawn concerning the mass-loss mechanism.

Dust and molecular shells in asymptotic giant branch stars - Mid-infrared interferometric observations of R Aql, R Aqr, R Hya, W Hya and V Hya