  The recently developed formalism of Markovian master equations for quantum
open systems with external periodic driving is applied to the theory of
dynamical decoupling by periodic control. This new approach provides a more
detailed quantitative picture of this phenomenon applicable both to traditional
NMR spectroscopy and to recent attempts of decoherence reduction in quantum
devices. For popular choices of bath spectral densities and a two-level system,
exact formulas for the relaxation rates are available. In the context of NMR
imaging, the obtained formulas allow the extraction of a new parameter
describing bath relaxation time from the experimental data.
