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.