Dynamics of an open system interacting with theenvironment considered as a thermostate may be formulatedin terms of a master equation with an integral operator allowing for the relaxation process, [Zwanzig 1960]. In some part- ular cases this operator hasashort-lastingkernel that enables one to consider therelaxation as a Markovian process and to obtainthe master equation inthe Lindblad form, [Lindblad 1976 (a)]. In some situations the memory effects become, however, important and the dynamics of thesystem gets much more involved, [Barnett 2001]. A similar situation arises inthe case where a set of consecutive or continuous measurements is performed. The purpose of this article is to consider a situation where some simplification of the generalform of the master equation with memory isstill possibleand the result isasimpler master equation. In particular, we consider the case of a dynamic system c- pled to a measured ancilla via a nondemolition interaction, [Caves 1980]. This simplifies the consideration essentiallywhereas providing an important special case inwhich the energy of the dynamic part is conserved.
We consider a composite quantum system consisting of a dynamic part - teracting with an ancillary part, the latter being subject to repeated projective measurements. The entire quantum system is assumed to evolve unitarily d- ing time ? t between the measurements. As a specific example, we analyze a harmonic oscillator coupledtoatwo-level ancillathat issubject to measu- ments.