Modulational stability and gap solitons of gapless systems: Continuous versus discrete limits

We consider a model consisting of two subsystems with crossing linear dispersion curves that describes, for example, light propagation in binary waveguide arrays. This model admits bright-dark gap solitons in both the discrete and the continuous (long-wavelength) limits, in spite of the absence of a gap in the linear (i.e., plane-wave) spectrum. We find that these solitons are always modulationally unstable in the continuous limit, whereas they can be stable in the discrete system if the amplitude of the background component exceeds a certain threshold.