Bound solitons in coupled nonlinear Schrödinger equations

Interaction of solitons belonging to different modes is analyzed in the framework of a system of nonlinear Schrödinger equations with incoherent and coherent nonlinear couplings with different group velocities. It is demonstrated that the two solitons can form a strongly bound state with coinciding centers and several weakly bound states with far-separated centers. The bound states of the latter type can be produced only by the coherent nonlinear coupling, provided it is stronger than the incoherent one. The results obtained are employed to explain qualitatively recent experiments with interactions of solitary pulses in the subcritical traveling-wave convection.