  The sensitivity of a repeated measurement-based purification scheme to
additional undesired couplings is analyzed, focusing on the very simple and
archetypical system consisting of two two-level systems interacting with a
repeatedly measured one. Several regimes are considered and in the strong
coupling (i.e., when the coupling constant of the undesired interaction is very
large) the occurrence of a quantum Zeno effect is proven to dramatically
jeopardize the efficiency of the purification process.
