Multi - instantons, supersymmetry and topological field theories

In this letter we argue that instanton-dominated Green's functions in N = 2 Super Yang-Mills theories can be equivalently computed either using the so-called constrained instanton method or making reference to the topological twisted version of the theory. Defining an appropriate BRST operator (as a supersymmetry plus a gauge variation), we also show that the expansion coefficients of the Seiberg-Witten effective action for the low-energy degrees of freedom can be written as integrals of total derivatives over the moduli space of self-dual gauge connections. (C) 2000 Elsevier Science B.V. All rights reserved.