  We analyze some solvable models of a quantum mechanical system in interaction
with a reservoir when the initial state is not factorized. We apply
Nakajima-Zwanzig's projection method by choosing a reference state of the
reservoir endowed with the mixing property. In van Hove's limit, the dynamics
is described in terms of a master equation. We observe that Markovianity
becomes a valid approximation for timescales that depend both on the form
factors of the interaction and on the observables of the reservoir that can be
measured.
