  The counting of long strings in ADS3, in the context of Type IIB string
theory on $ADS_3 \times S^3 \times T^4$, is used to exhibit the action of the
duality group $O(5,5;Z)$, and in particular its Weyl Subgroup $S_5 \bowtie
Z_2$, in the non-perturbative phenomena associated with continuous spectra of
states in these backgrounds. The counting functions are related to states in
Fock spaces. The symmetry groups also appear in the structure of
compactifications of instanton moduli spaces on $T^4$.
