A custoidal symmetry for the cosmological constant in the effective theories of four-dimensional superstrings

We discuss supersymmetry breaking in the effective supergravity theories of four-dimensional N=1 superstrings. Improving a recent suggestion, we introduce a superpotential modification that describes spontaneous supersymmetry breaking with vanishing cosmological constant and Str M2 = 0 at any minimum of the tree level potential. We prove that in a class of models there are classical minima at which also Str f(M2) = 0 for any function f. In particular, the whole one-loop cosmological constant vanishes. We propose a new boson-fermion symmetry of the spectrum, relating the graviton and the 'dilatino', which explains these remarkable properties. (orig.)