  In the framework of theory of open quantum systems, we derive quantum master
equations for the ultrastrong system-bath coupling regime and, more generally,
the strong-decoherence regime. In this regime, the strong decoherence is
complemented by slow relaxation processes. We use a generalization of the
Foerster and modified Redfield peturbation theories known in theory of
excitation energy transfer. Also, we show that the mean force Gibbs state in
the corresponding limits are stationary for the derived master equations.
