  We describe physical phenomena associated with a class of transitions that
occur in the study of supersymmetric three-cycles in Calabi-Yau threefolds. The
transitions in question occur at real codimension one in the complex structure
moduli space of the Calabi-Yau manifold. In type IIB string theory, these
transitions can be used to describe the evolution of a BPS state as one moves
through a locus of marginal stability: at the transition point the BPS particle
becomes degenerate with a supersymmetric two particle state, and after the
transition the lowest energy state carrying the same charges is a
non-supersymmetric two particle state. In the IIA theory, wrapping the cycles
in question with D6-branes leads to a simple realization of the Fayet model:
for some values of the CY modulus gauge symmetry is spontaneously broken, while
for other values supersymmetry is spontaneously broken.
