N=2 WORLDSHEET INSTANTONS YIELD CUBIC SELF-DUAL YANG-MILLS

When the gauge instantons on the N = 2 string worldsheet are properly included in the sum over topologies, the breaking of SO(2,2) Lorentz s ymmetry in R-2,R-2 is parametrized by a spacetime twister containing t he string coupling and theta angle. The resulting (tree-level) effecti ve action for the open string is not Yang's but Leznov's cubic action for self-dual Yang-Mills in a light-cone gauge. In the closed case, Pl ebanski's action for self-dual gravity gets modified analogously. In c ontrast to the N = 1 NSR string, picture-changing is not locally inver tible, but produces a semi-infinite tower of massless physical states with ever-increasing spin, perhaps related to harmonic superspace. A t runcation yields the two-field action of Chalmers and Siegel. (C) 1997 Published by Elsevier Science B.V.