Behaviour of the Absorptive Part of the W Electromagnetic Vertex
  The absorptive part of the $WW\gamma$ vertex induced by massive fermion loops is considered for different kinematical configurations. We show that the axial part of this vertex is different from zero not only when massive fermions are involved but also for massless fermion loops, if one of the W bosons is space-like and the other is time-like. We also discuss in what sense Low's soft photon theorem is satisfied. 
M. Beuthe, R. Gonzalez Felipe, G. Lopez Castro, J. Pestieau