Some years ago, Willems & de Jong (1988) noticed that many carbon stars display an excess of emission at 60 µm and explained it by the presence of a fossil dust shell, containing only cold dust. This detached dust shell would be the result of an interruption of the mass loss, consequence of a thermal pulse. Detached shells around C stars have actually been mapped in the CO lines (Olofsson et al. 1992), and at 60µm (Waters et al. 1994). In 1992, Zijlstra et al. found about 100 M stars displaying an excess of emission at 60 µm, and proposed that interruptions of the mass loss due to thermal pulses is a general phenomenon on the AGB. This assumption is now supported by the theoretical calculations of Vassiliadis & Wood (1993). Here we present a detailed study of the 100 M stars of Zijlstra et al. in order to test the previous assumption.