Consequences of t-channel unitarity for gamma(*)p and gamma(*) gamma(*) amplitudes

We show that t-channel unitarity constraints make it possible to obtain the photon-photon cross sections from the photon-proton and proton-proton cross sections, for Q2<150 GeV2. In order to do so, one must postulate the existence of double-pole or triple-pole singularities in the complex j plane.