  Shells are fine stellar structures identified by their arc-like shapes
present around a galaxy and currently thought to be vestiges of galaxy
interactions and/or mergers. The study of their number, geometry, stellar
populations and gas content can help to derive the interaction/merger history
of a galaxy. Numerical simulations have proposed a mechanism of shell formation
through phase wrapping during a radial minor merger. Alternatively, there could
be barely a space wrapping, when particles have not made any radial oscillation
yet, but are bound by their radial expansion, or produce an edge-brightened
feature. These can be distinguished, because they are expected to keep a high
radial velocity. While shells are first a stellar phenomenon, HI and CO
observations have revealed neutral gas associated with shells. Some of the gas,
the most diffuse and dissipative, is expected to be driven quickly to the
center if it is travelling on nearly radial orbits. Molecular gas, distributed
in dense clumps, is less dissipative, and may be associated to shells, and
determine their velocity, too difficult to obtain from stars. We present here a
search for molecular gas in nine shell galaxies with the IRAM-30m telescope.
Six of them are detected in their galaxy center, and in three galaxies, we
clearly detect molecular gas in shells. The derived amount of molecular gas
varies from 1.5 10$^8$ to 3.4 10$^9$ M$_\odot$ in the shells. For two of them
(Arp 10 and NGC 3656), the shells are characteristic of an oblate system. Their
velocity is nearly systemic, and we conclude that these shells are
phase-wrapped. For the third one (NGCB3934) the shells appear to participate to
the rotation, and follow up with higher spatial resolution is required to
conclude.
