  We derive general expressions for the current and shot noise, taking into
account non-Markovian memory effects. In generalization of previous approaches
our theory is valid for arbitrary Coulomb interaction and coupling strength and
is applicable to quantum dots and more complex systems like molecules. A
diagrammatic expansion up to second-order in the coupling strength, taking into
account co-tunneling processes, allows for a study of transport in a regime
relevant to many experiments. As an example, we consider a single-level quantum
dot, focusing on the Coulomb-blockade regime. We find super-Poissonian shot
noise due to spin-flip co-tunneling processes at an energy scale different from
the one expected from first-order calculations, with a sensitive dependence on
the coupling strength.
