  The nature of the mechanism responsible for producing the spectacular,
geometrically thin, spherical shells found around some carbon stars has been an
enigma for some time. Based on extensive radiative transfer modelling of both
CO line emission and dust continuum radiation for all objects with known
detached molecular shells, we present compelling evidence that these shells
show clear signs of interaction with a surrounding medium. The derived masses
of the shells increase with radial distance from the central star while their
velocities decrease. A simple model for interacting winds indicate that the
mass-loss rate producing the faster moving wind has to be almost two orders of
magnitudes higher (~10^-5 solar masses per year) than the slower AGB wind (a
few 10^-7 solar masses per year) preceding this violent event. At the same
time, the present-day mass-loss rates are very low indicating that the epoch of
high mass-loss rate was relatively short, on the order of a few hundred years.
This, together with the number of sources exhibiting this phenomenon, suggests
a connection with He-shell flashes (thermal pulses). We report the detection of
a detached molecular shell around the carbon star DR Ser, as revealed from new
single-dish CO (sub-)millimetre line observations. The properties of the shell
are similar to those characterising the young shell around U Cam.
