Gauge invariance in two-particle scattering
  It is shown how gauge invariance is obtained for the coupling of a photon to a two-body state described by the solution of the Bethe-Salpeter equation. This is illustrated both for a complex scalar field theory and for interaction kernels derived from chiral effective Lagrangians. 
B. Borasoy, P.C. Bruns, U.-G. Mei{\ss}ner and R. Ni{\ss}ler