  String theory on AdS_3 x S^3 is studied in the hybrid formulation. We give a
detailed description of the PSL(2|2) supergroup WZW model that underlies the
background with pure NS-NS flux, and determine the BRST-cohomology
corresponding to the massless string states. The resulting spectrum is shown to
match exactly with the expected supergravity answer, including the sectors with
small KK momentum on the sphere.
