The dynamics of Rydberg wave packets in NO are investigated in the regime where the electronic period is comparable with the rotational motion of the molecular ion core. The presence of a rotating molecular core manifests itself in the wave packet dynamics as a series of peaks separated by the rotational beat period T-Rot, but offset by DeltamuT(Rot), where Deltamu is the difference in quantum defect between the two dominant Rydberg series in the superposition. We rationalize this by treating the dynamics of a wave packet created from a coherent superposition of two interleaved Rydberg series as two separate electron wave packets, which interfere with one another when they overlap spatially. There is a periodic phase difference between the two wave packets that depends on the rotational energy of the core in each Rydberg series and also on the quantum defects. The resulting interference pattern in the Rydberg population manifests itself as peaks in the wave packet spectrum at the stroboscopic period. (C) 2003 American Institute of Physics.