  Suppose that an algebraic torus $G$ acts algebraically on a projective
manifold $X$ with generically trivial stabilizers. Then the Zariski closure of
the set of pairs $\{(x,y)\in X\times X\mid y=gx \text{for some}g\in G\}$
defines a nonzero equivariant cohomology class $[\Delta_G]\in H^*_{G\times
G}(X\times X)$. We give an analogue of this construction in the case where $X$
is a compact symplectic manifold endowed with a hamiltonian action of a torus,
whose complexification plays the role of $G$. We also prove that the Kirwan map
sends the class $[\Delta_G]$ to the class of the diagonal in each symplectic
quotient. This allows to define a canonical right inverse of the Kirwan map.
