  In this note we compute exact boundary spectra for D-instantons in
sigma-models on the supergroup PSL(2|2). Our results are obtained through an
explicit summation of the perturbative expansion for conformal dimensions to
all orders in the curvature radius. The analysis exploits several remarkable
properties of the perturbation series that arises from rescalings of the metric
on PSL(2|2) relative to a fixed Wess-Zumino term. According to Berkovits, Vafa
and Witten, the models are relevant in the context of string theory on AdS3
with non-vanishing RR-flux. The note concludes with a number of comments on
various possible generalizations to other supergroups and higher dimensional
supercoset theories.
