  It is shown, by means of a simple specific example, that for integrable
systems it is possible to build up approximate eigenfunctions, called {\it
asymptotic eigenfunctions}, which are concentrated as much as one wants to a
classical trajectory and have a lifetime as long as one wants. These states are
directly related to the presence of shell structures in the quantal spectrum of
the system. It is argued that the result can be extended to classically chaotic
system, at least in the asymptotic regime.
