Twist-3 contribution to the $\gamma^*\gamma\to \pi\pi$ amplitude in the   Wandzura-Wilczek approximation
  We have calculated the Wandzura-Wilczek contribution to the twist-3 part of $\gamma^*\gamma\to 2\pi$ amplitude. It describes interaction of the longitudinally polarized virtual photon with the real one, and it is suppressed by 1/Q, where $Q^2$ is the virtuality of the $\gamma^*$, as compared to the twist-2 contribution. We have found that, in the Wandzura-Wilczek approximation, factorization applies to the twist-3 amplitude. 
N. Kivel, L. Mankiewicz