  We consider a driven 2-level system with one level showing spontaneous decay
to an otherwise uncoupled third level. Rabi transitions to the unstable level
are strongly damped. This simple configuration can be used to demonstrate and
to explore the quantum Zeno effect leading to a freezing of the system to the
initial level. A comparison with repeated projection measurements is given. A
treatment within a phenomenological theory of continuous measurements is
sketched. The system visualizes the important role of null measurements
(negative result measurements) and may serve as a good example for a truly
continuous measurement.
