production at NNLO: the flavour off-diagonal channels.

We consider QCD radiative corrections to the associated production of a heavy-quark pair ( ) with a generic colourless system at hadron colliders. We discuss the resummation formalism for the production of the system at small values of its total transverse momentum . We present the results of the corresponding resummation coefficients at next-to-leading and, partly, next-to-next-to-leading order.