  A system of metastable plus unstable states is discussed. The mass matrix
governing the time development of the system is supposed to vary slowly with
time. The adiabatic limit for this case is studied and it is shown that only
the metastable states obtain the analogs of the dynamical and geometrical phase
factors familiar from stable states. Abelian and non-abelian geometric phase
factors for metastable states are defined.
