Perturbative and Non-Perturbative Partial Supersymmetry Breaking: $N=4 \to N=2 \to N=1$

We show the existence of a supersymmetry breaking mechanism in string theory, where N=4 supersymmetry is broken spontaneously to N=2 and N=1 with moduli dependent gravitino masses. The spectrum of the spontaneously broken theory with lower supersymmetry is in one-to-one correspondence with the spectrum of the heterotic N=4 string. The mass splitting of the N=4 spectrum depends on the compactification moduli as well as the three R-symmetry charges. In the large moduli limit a restoration of the N=4 supersymmetry is obtained. As expected the graviphotons and some of the gauge bosons become massive in N=1 vacua. At some special points of the moduli space some of the N=4 states with non-zero winding numbers and with spin 0 and {1/2} become massless chiral superfields of the unbroken N=1 supersymmetry. Such vaccua have a dual type II description, in which there are magnetically charged states with spin 0 and {1/2} that become massless. The heterotic-type II duality suggests some novel non-perturbative transitions on the type II side. Such transitions do not seem to have a geometric interpretation, since they relate type II vaccua with symmetric worlsheet structure to assymetric ones. The heteroric interpretation of such a transition is an ordinary Higgsing of an SU(2) factor. In the case of N=4 \to N=2, the perturbative N=2 prepotential is determined by the perturbative N=4 BPS states. This observation let us to suggest a method which determines the exact non-perturbative prepotential of the effective N=2 supergravity using the shifted spectrum of the non-perturbative BPS states of the underlying N=4 theory.