The massless string spectrum on AdS_3 x S^3 from the supergroup
  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. 
Matthias R. Gaberdiel, Sebastian Gerigk
hep-th