Consequences of t-channel unitarity for the interaction of real and   virtual photons at high energies
  We analyze the consequences of t-channel unitarity for photon cross sections and show what assumptions are necessary to allow for the existence of new singularities at $ Q^{2}=0 $ for the $ \gamma p $ and $ \gamma \gamma $ total cross sections. For virtual photons, such singularities can in general be present, but we show that, apart from the perturbative singularity associated with $ \gamma ^{*}\gamma ^{*}\to q\bar q $, no new ingredient is needed to reproduce the data from LEP and HERA, in the Regge region. 
E. Martynov, J.R. Cudell, G. Soyez