Gauge invariant action for superstring in Ramond-Ramond plane-wave   background
  We present a gauge invariant action for a superstring in the plane wave background with Ramond-Ramond (RR) five-form flux. The Wess-Zumino term is given explicitly in a bilinear form of the left invariant currents by introducing a fermionic center to define the nondegenerate group metric. The reparametrization invariance generators, whose combinations are conformal generators, and fermionic constraints, half of which generate kappa-symmetry, are obtained. Equations of motion are obtained in conformal invariant and background covariant manners. 
Machiko Hatsuda, Kiyoshi Kamimura and Makoto Sakaguchi