  We consider here renormalizable theories without relevant couplings and
present an I.R. consistent technique to study corrections to short distance
behavior (Wilson O.P.E. coefficients) due to a relevant perturbation. Our
method is the result of a complete reformulation of recent works on the field,
and is characterized by a more orthodox treatment of U.V. divergences that
allows for simpler formulae and consequently an explicit all order
(regularization invariant) I.R. finitess proof. Underlying hypotheses are
discussed in detail and found to be satisfied in conformal theories that
constitute a natural field of application of this approach.
