  We reconsider the influence of two-particle final state interactions (FSI) on
two-particle Bose-Einstein interferometry. We concentrate in particular on the
problem of particle emission at different times. Assuming chaoticity of the
source, we derive a new general expression for the symmetrized two-particle
cross section. We discuss the approximations needed to derive from the general
result the Koonin-Pratt formula. Introducing a less stringent version of the
so-called smoothness approximation we also derive a more accurate formula. It
can be implemented into classical event generators and allows to calculate FSI
corrected two-particle correlation functions via modified Bose-Einstein
"weights".
