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On the upper AGB, stellar evolution is determined by mass loss. It is now well established from observations that mass loss is subject to large temporal variations, or interruptions (Willems and de Jong 1988, A&A 196, 173; Zijlstra et al. 1992, A&A 265, L5), probably related to the luminosity changes that occur in thermally pulsating AGB stars. As a natural consequence of such mass loss variability, detached shells are expected to develop repeatedly. The existence of detached shells around C-stars is not only indicated by their spectral energy distributions but has been confirmed by spatially resolved CO maps (Olofsson et al. 1996, A&A 311, 587) and some high-resolution IRAS images (Waters et al. 1994, A&A 281, L1). However, comparable observations of detached shells around O-rich AGB stars have not been obtained up to now, suggesting that this phenomenon is less frequent or less pronounced for these objects. If confirmed, such a situation would imply significant differences in the mass loss behavior between C-rich and O-rich AGB stars. The present proposal is dedicated to address this question by searching for detached dust shells around O-rich objects and analyzing their properties in comparison to those of C-rich stars. Guided by our hydrodynamical models which show that the existence of a cold detached dust shell around O-rich sources is correlated with weak or almost non-existing silicate features (see enclosed Fig. 1), we propose a sample of five promising candidates (3 of them known to be extended) for observation by ISO. We supplemented our sample by 3 new C-star candidates (also known to be extended) showing significant 60 and 100 micron (um) excess fluxes after correction for extension. Data analysis will make use of elaborate time-dependent radiation hydrodynamics models of models of AGB winds which shall serve as an important new tool for a physical interpretation of the infrared spectra and spatially resolved images. Our selection of targets gives us the possibility to investigate the mass loss history in both types of AGB stars (O- and C-rich) in a comparative study and should improve our knowledge about mass loss processes on the upper AGB and may shed some light on the problem of C-star formation.