  We consider a finite-dimensional quantum system coupled to the bosonic
radiation field and subject to a time-periodic control operator. Assuming the
validity of a certain dynamic decoupling condition we approximate the system's
time evolution with respect to the non-interacting dynamics. For sufficiently
small coupling constants $g$ and control periods $T$ we show that a certain
deviation of coupled and uncoupled propagator may be estimated by
$\mathcal{O}(gt \, T)$. Our approach relies on the concept of Kato stability
and general theory on non-autonomous linear evolution equations.
