http://arxiv.org/abs/hep-th/0403199 Parity Invariance For Strings In Twistor Space Authors: Edward Witten Comments: 18 pp Topological string theory with twistor space as the target makes visible some otherwise difficult to see properties of perturbative Yang-Mills theory. But left-right symmetry, which is obvious in the standard formalism, is highly unclear from this point of view. Here we prove that tree diagrams computed from connected $D$-instanton configurations are parity-symmetric. The main point in the proof also works for loop diagrams Is this the last paper of Witten about strings before he renounces to string theory? oops sorry for the title. I meant "twistor"