  We propose a method to study the thermodynamic behaviour of small systems
beyond the weak coupling and Markovian approximation, which is different in
spirit from conventional approaches. The idea is to redefine the system and
environment such that the effective, redefined system is again coupled weakly
to Markovian residual baths and thus, allows to derive a consistent
thermodynamic framework for this new system-environment partition. To achieve
this goal we make use of the reaction coordinate mapping, which is a general
method in the sense that it can be applied to an arbitrary (quantum or
classical and even time-dependent) system coupled linearly to an arbitrary
number of harmonic oscillator reservoirs. The core of the method relies on an
appropriate identification of a part of the environment (the reaction
coordinate), which is subsequently included as a part of the system. We
demonstrate the power of this concept by showing that non-Markovian effects can
significantly enhance the steady state efficiency of a three-level-maser heat
engine, even in the regime of weak system-bath coupling. Furthermore, we show
for a single electron transistor coupled to vibrations that our method allows
one to justify master equations derived in a polaron transformed reference
frame.
