We propose to observe the outer regions of 3 elliptical/S0
 galaxies with optical/HI shells in the FIR with Isophot in
 order find evidence for the presence of cool dust in the
 shells. A large fraction of elliptical and early type galaxies
 exhibit shells, possibly caused by interactions/merging with
 gas rich companions. We have found HI associated with the faint
 optical shells in 4 systems, the most prominent examples being
 Centaurus A and NGC 2865. The kinematics of the HI cast doubt on
 several of the existing scenarios involving both internal and
 external origins for the outer shells. The external scenarios
 appear to have the most promise at the moment.

 The external scenarios describe infall of a companion object
 into the parent galaxy. This could be either a gas rich companion
 galaxy or primordial gas clouds. A way to discriminate between
 these possibilities is to search for the presence of dust in these
 outer regions where we found the HI. Pointed observations with
 Isophot at 135 and 200 micron will go a factor 10 deeper than
 the existing IRAS data where these shells have not been detected
 at a level of about 1 MJy/ster. These observations will not only
 provide clues about the presence of dust, but in the case of
 detection of emission also provide a measure of the FIR energy
 distribution and hence the temperature of the dust, which we
 expect to be low.