The D1/D5 system and singular CFT

We study the conformal field theory of the D1/D5 system compactified on X ( X is T-4 or K3). It is described by a sigma model whose target space is the moduli space of instantons on X. For values of the parameters where the br anes can separate, the spectrum of dimensions in the conformal field theory exhibits a continuum above a gap. This continuum leads to a pathology of t he conformal field theory, which explains a variety of problems in various systems. In particular, we explain the apparent discrepancy between differe nt methods of finding the spectrum of chiral fields at certain points in th e moduli space of the system.