  We present an exact expansion of the master equation for an open quantum
system. The resulting equation is time local and enables us to calculate
clearly defined higher order corrections to the Born-Markov approximation. In
particular, we show that non-Markovian terms are of the same order of magnitude
as higher order terms in the system-bath coupling. As a result we emphasize
that analyzing non-Markovian behaviour of a system implies going beyond Born
approximation. Additionally, we address with this approach the initial state
problem occurring in non-Markovian master equations.
