  Multiple shell planetary nebulae (MSPNe) are most likely a consequence of the
modulation of the mass-loss rate during the thermal pulses on the AGB. By using
numerical simulations of their formation, the models' predictions and the
history of the winds can be investigated. As their halos have low densities and
expansion velocities, MSPNe are expected to be affected by interaction with the
ISM . This would then give us evidence of the local ISM conditions.
  In order to study the formation of MSPNe, we have performed numerical
simulations following the evolution of the stellar winds for a 1 solar mass
star from the AGB to the post-AGB stages. Without invoking any asymmetry for
the stellar wind and taking into account the effects of a moving central star,
an asymmetric halo is formed as a consequence of the interaction with the ISM.
  We found that the asymmetries caused by the interaction take place from the
beginning of the evolution and have an enormous influence on the formation of
the halo.
