The D1/D5 system and singular CFT

We study the conformai field theory of the D1/D5 system compactified on X (X is T⁴ 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 branes can separate, the spectrum of dimensions in the conformai field theory exhibits a continuum above a gap. This continuum leads to a pathology of the conformai field theory, which explains a variety of problems in various systems. In particular, we explain the apparent discrepancy between different methods of finding the spectrum of chiral fields at certain points in the moduli space of the system.