Wandzura-Wilczek approximation from generalized rotational invariance

We present the alternative way of derivation of the Wandzura-Wilczek relations between the kinematical twist-3 and twist-2 functions, parameterizing hadronic matrix element in two-photon processes gamma*pi --> gamma pi and gamma*gamma --> pi pi. The new equations, providing the independence of the physical cross-sections on the choice of the light-cone direction, are suggested and explored. The amplitude of gamma*gamma --> pi pi up to genuine twist-3 accuracy is found. (C) 2001 Published by Elsevier Science B.V.