  We present a variant of Mourre's commutator theory. We apply it to prove the
limiting absorption principle for Schr\"odinger operators with a perturbed
Wigner-Von Neumann potential at suitable energies. To our knowledge, this
result is new since we allow a long range pertubation of the Wigner-Von Neumann
potential. Furthermore, we can show that the usual Mourre theory, based on
differential inequalities and on the generator of dilations, cannot apply to
our Schr\"odinger operators.
