  In the hybrid formulation of string theory on AdS_3 x S^3, the
compactification-independent physical spectrum at the first massive level is
determined. We find that these states transform in representations of the Lie
superalgebra psl(2|2) and present a cohomological characterisation of them
within the context of the PSL(2|2) WZW model. We conjecture that a similar
cohomological description of the physical states also exists at higher mass
levels and give partial evidence for this by comparing our results to the RNS
string spectrum.
