Power corrections of off-forward quark distributions and harmonic operators with definite twist

A group theoretical procedure for parametrizing non-forward matrix elements of non-local QCD operators is introduced. The related (two-variable) distribution amplitudes are given as sum over power corrections of the double distributions. For totally symmetric (local) operators the infinite twist decomposition is given completely and for operators with non-trivial symmetry type up to twist 3. The power corrections for most of the phenomenological relevant double distributions and the vector meson wave functions are determined. They may be obtained, by harmonic extension, from the corresponding operators or distribution amplitudes on the light-cone.