The Donnachie-Landshoff pomeron and gauge invariance
  The pomeron model of Donnachie and Landshoff does not conserve the electromagnetic current when applied to diffractive reactions such as electroproduction of a quark-antiquark pair or of a vector meson. We propose a treatment of this problem which ensures a physical behaviour of cross sections in the photoproduction limit and show that it leads to results rather similar to those obtained from two-gluon exchange. 
M. Diehl