M-theory pp-waves, Penrose limits and supernumerary supersymmetries

We study supersymmetric pp-waves in M-theory, their dimensional reduction to D0-branes or pp-waves in type IIA, and their T-dualisation to solutions in the type 1113 theory. The general class of pp-waves that we consider encompass the Penrose limits of AdS(p) X S-q with (p, q) = (4, 7), (7,4), (3,3), (3,2), (2,3), (2,2), but includes also many other examples that can again lead to exactly-solvable massive strings, but which do not arise from Penrose limits. All the pp-waves in D = 11 have 16 "standard" Killing spinors, but in certain cases one finds additional, or "supernumerary," Killing spinors too. These give rise to linearly-realised supersymmetries in the string or matrix models. A focus of our investigation is on the circumstances when the Killing spinors are independent of particular coordinates (x(+) or transverse-space coordinates), since these will survive at the field-theory level in dimensional reduction or T-dualisation. (C) 2002 Elsevier Science B.V. All rights reserved.