  In this contribution it is considered a simple and solvable model consisting
of a qubit in interaction with an oscillator exposed to a very small
"environment" (a second qubit). An isolated qubit-oscillator system having the
oscillator initially in one of its energy eigenstates exhibits Rabi
oscillations, an evidence of coherent quantum behaviour. It is shown here in
which way the coupling to a small "environment" disrupts such regular
behaviour, leading to a quasi-periodic dynamics for the qubit linear entropy.
In particular, it is found that the linear entropy is very sensitive to the
amount of mixedness of the "environment". For completeness, fluctuations in the
oscillator energy are also taken into account.
