  We develop a coupled-mode theory for spatial gap solitons in the
one-dimensional photonic lattices induced by interfering optical beams in a
nonlinear photorefractive crystal. We derive a novel system of coupled-mode
equations for two counter-propagating probe waves, and find its analytical
solutions for stationary gap solitons. We also predict the existence of moving
(or tilted) gap solitons and study numerically soliton collisions.
