  We use standard distorted wave theory techniques and dimensional
regularization to find out solutions of the nucleon-nucleon Lippman--Schwinger
equation with a kernel determined by the Weinberg's next-to-leading potential,
which consists of one--pion exchange and additional contact terms with
derivatives. Though for simplicity, we restrict the discussion to the $^1S_0$
channel and to contact terms containing up to two derivatives, the
generalization to higher waves and/or number of derivatives is straightforward.
The undetermined low energy constants emerging out of the renormalization
procedure are fitted to data.
