Calculation of two-photon exchange effects for $\Delta$ production in   electron-proton collisions
  We present an evaluation of a two-photon exchange correction to the cross section for unpolarized $\Delta$ isobar production in electron-proton collisions, using a relativistic, crossing symmetric and gauge invariant approach. The calculated box and crossed-box diagrams include nucleon and $\Delta$ intermediate states. We find a relation between the angular nonlinearity of the two-photon exchange contribution and the value of the $\gamma \Delta \Delta$ coupling constant. 
S. Kondratyuk, P. G. Blunden