The Ground Ring of N = 2 Minimal String Theory

We study the N = 2 string theory or the N = 4 topological string on the deformed CHS background. That is, we consider the N = 2 minimal model coupled to the N = 2 Liouville theory. This model describes holographically the topological sector of Little String Theory. We use degenerate vectors of the respective N = 2 Verma modules to find the set of BRST cohomologies at ghost number zero–the ground ring, and exhibit its structure. Physical operators at ghost number one constitute a module of the ground ring, so the latter can be used to constrain the S-matrix of the theory. We also comment on the inequivalence of BRST cohomologies of the N = 2 string theory in different pictures.