We consider the effect of a spatially periodic forcing at the onset of one-dimensional periodic patterns in nonequilibrium systems. It is shown that the competition between periodicities can lead to a commensurate-incommensurate transition mediated by solitons as in equilibrium systems. This result is in agreement with a recent experiment.

• Received 30 September 1985

DOI:https://doi.org/10.1103/PhysRevLett.56.724