IIB PP-Waves with Extra Supersymmetries

We examine Killing spinor equations of the general IIB pp-wave backgrounds, which contain a scalar H(x^m,x^-) in the metric and a self-dual four-form \xi(x^m,x^-) in the self-dual five-form flux. Considering non-harmonic extra Killing spinors, we find that if the backgrounds admit at least one extra Killing spinor in addition to 16 standard Killing spinors, backgrounds can be reduced to the form with H=A_{mn}(x^-)x^mx^n and \xi(x^-), modulo coordinate transformations. We examine further the cases in which the extra Killing spinors are characterized by a set of Cartan matrices. Solving Killing spinor equations, we find IIB pp-wave backgrounds which admit 18, 20, 24 and 32 Killing spinors.