On the (0,4) Conformal Field Theory of the Throat
  In SO(32) heterotic string theory, the space-time at the core of N coincident NS-fivebranes is an infinite throat, R x S^3. As shown by Witten, the throat signals a singularity in the usual heterotic string conformal field theory and a non--perturbative USp(2N) gauge group appears, due to the N small instantons at the fivebranes' core. Nevertheless, we look for some trace of the non-perturbative physics in a description of the heterotic string infinitely far down the throat. Our guide is a D1-brane probing N D5-branes in type I, which yields a 1+1 dimensional (0,4) supersymmetric model with ADHM data in its couplings, as shown by Douglas. The neighbourhood of the classical boundary of the hypermultiplet moduli space of the theory flows to an exact conformal field theory description of the throat theory. Ironically, the remnant of the non-perturbative symmetry is indeed found in the conformal field theory, lurking in the structure of the partition function, and encoded in a family of deformations of the theory along flat directions. The deformations have an explicit description using the flow from the type I theory, and have a hyperKahler structure. Similar results hold true for the analogous (4,4) supersymmetric situation in the type IIB theory, as is evident in the work of Diaconescu and Seiberg. 
Clifford V. Johnson