Aspects of N-T>=2 topological gauge theories and D-branes

We comment on various aspects of topological gauge theories possessing N-T greater than or equal to 2 topological symmetry: (1) We show that the construction of Vafa-Witten and Dijkgraaf-Moore of 'balanced' topological field theories is equivalent to an earlier construction in terms of N-T=2 superfields inspired by supersymmetric quantum mechanics. (2) We explain the relation between topological field theories calculating signed and unsigned sums of Euler numbers of moduli spaces, (3) We show that the topological twist of N=4 d=4 Yang-Mills theory recently constructed by Marcus is formally a deformation of four-dimensional super-BF theory, (4) We construct a novel N-T=2 topological twist of N=4 d=3 Yang-Mills theory, a 'mirror' of the Casson invariant model, with certain unusual features (e.g. no bosonic scalar field and hence no underlying equivariant cohomology) (5) We give a complete classification of the topological twists of N=8 n=3 Yang-Mills theory and show that they are realised as world-volume theories of Dirichlet two-brane instantons wrapping supersymmetric three-cycles of Calabi-Yau three-folds and G(2)-holonomy Joyce manifolds. (6) We describe the topological gauge theories associated to D-string instantons on holomorphic curves in K3's and Calabi-Yau 3-folds. (C) 1997 Elsevier Science B.V.