  We consider wavepackets composed of two modulated carrier Bloch waves with
opposite group velocities in the one dimensional periodic Nonlinear
Schroedinger/Gross-Pitaevskii equation. These can be approximated by first
order coupled mode equations (CMEs) for the two slowly varying envelopes. Under
a suitably selected periodic perturbation of the periodic structure the CMEs
possess a spectral gap of the corresponding spatial operator and allow families
of exponentially localized solitary waves parametrized by velocity. This leads
to a family of approximate solitary waves in the periodic nonlinear
Schroedinger equation. Besides a formal derivation of the CMEs a rigorous
justification of the approximation and an error estimate in the supremum norm
are provided. Several numerical tests corroborate the analysis.
