  Certain one-loop processes in eleven-dimensional supergravity compactified on
T**2 determine exact, non-perturbative, terms in the effective action of type
II string theories compactified on a circle. One example is the modular
invariant U(1)-violating interaction of sixteen complex spin-1/2 fermions of
ten-dimensional type IIB theory. This term, together with the (curvature)**4
term, and many other terms of the same dimension are all explicitly related by
supersymmetry.
