  We define fermionic collective coordinates for type-IIB Dirichlet instantons
and discuss some effects of the associated fermionic zero modes within the
dilute gas framework. We show that the standard rules for clustering of zero
modes in the dilute limit, and the fermion-exchange interactions follow from
world-sheet Ward identities. Fermion exchange is strongly attractive at
string-scale distances, which makes the short-distance Hagedorn singularity
between instantons and anti-instantons even stronger.
