Right, thanks.. I was too worried about the details and forgot to double check the potential I guess.
##\int \frac{GMm}{r^2} dr = -\frac{GMm}{r} = -\frac{\mu m}{r}##
of course. Thanks a lot, now everything works out :-)
This isn't exactly homework, but something which you'd get as an assignment, so I'll still post it here in order to reach the right people..
I'm attempting to freshen up my knowledge on Hamiltonian systems, so I've tried to formulate the Kepler problem in Hamiltonian dynamics...