Non-local symmetries of the closed N=2 string
By carefully analysing the picture-dependence of the BRST cohomology an infinite set of symmetry charges of the closed N=2 string is identified. The transformation laws of the physical vertex operators are shown to coincide with the linearised non-local symmetries of the Plebanski equation (which is the effective field theory of the closed N=2 string). Moreover, the corresponding Ward identities are powerful enough to allow for a rederivation of the well known vanishing theorem for the tree-level correlation functions with more than three external legs.