N=4 Superconformal Symmetry for the Covariant Quantum Superstring
  We extend our formulation of the covariant quantum superstring as a WZNW model with N=2 superconformal symmetry to N=4. The two anticommuting BRST charges in the N=4 multiplet of charges are the usual BRST charge Q_S and a charge Q_V proposed by Dijkgraaf, Verlinde and Verlinde for topological models. Using our recent work on "gauging cosets", we then construct a further charge Q_C which anticommutes with Q_S + Q_V and which is intended for the definition of the physical spectrum. 
P.A. Grassi (DISTA, Univ. Piem Orien., YITP Stony Brook) and P. van   Nieuwenhuizen (YITP, Stony Brook)