Equivariant Topological Sigma Models

We identify and examine a generalization of topological sigma models suitable for coupling to topological open strings. The targets are Kahler manifolds with a real structure, i.e. with an involution acting as a complex conjugation, compatible with the Kahler metric. These models satisfy axioms of what might be called ``equivariant topological quantum field theory,'' generalizing the axioms of topological field theory as given by Atiyah. Observables of the equivariant topological sigma models correspond to cohomological classes in an equivariant cohomology theory of the targets. Their correlation functions can be computed, leading to intersection theory on instanton moduli spaces with a natural real structure. An equivariant $CP^1\times CP^1$ model is discussed in detail, and solved explicitly. Finally, we discuss the equivariant formulation of topological gravity on surfaces of unoriented open and closed string theory, and find a $Z_2$ anomaly explaining some problems with the formulation of topological open string theory.Comment: 39 pages, 6 figures (available upon request); pure LaTeX file. [[The paper was originally published as Prague preprint PRA-HEP-90/18 in December 1990. As the LaTeX file is still in demand, this is the original version with just some minor changes and misprint corrections; a Note has been added to update the state of the art.]