noticed first by Beskow and Nilsson [@BN]. It was only decade later, however, when Misra and Sudarshan [@MS] caught the imagination of the community by linking the effect to the well-known Zeno aporia about a flying arrow. While at first the subject was rather academical, in recent years the possibility of observing Zeno-type effects experimentally has become real and at present there are scores of physical papers discussing this topic.

On the mathematical side, the first discussion of the continuous observation appeared in [@Fr]. Two important questions, however, namely the existence of Zeno dynamics and the form of its effective Hamiltonian have been left open both in this paper and later in [@MS]. The second problem is particularly important when the subspace into which the state of the system is repeatedly reduced has dimension larger than one. A partial answer was given in [@Ex Sec. 2.4] where it was shown that the results of Chernoff [@Ch1; @Ch2] allow to determine the generator of the Zeno time evolution naturally through the appropriate quadratic form.

Our interest to the problem was rekindled by a recent paper by Facchi et al. [@FP] who studied the important special case when the presence of a 