  We describe a new class of instanton effects in string compactifications that
preserve only N=1 supersymmetry in four dimensions. As is well-known,
worldsheet or brane instantons in such a background can sometimes contribute to
an effective superpotential for the moduli of the compactification. We
generalize this phenomenon by showing that such instantons can also contribute
to new multi-fermion and higher-derivative F-terms in the low-energy effective
action. We consider in most detail the example of heterotic compactification on
a Calabi-Yau threefold X with gauge bundle V, in which case we study worldsheet
instanton effects that deform the complex structure of the moduli space
associated to X and V. We also give new, slightly more economical derivations
of some previous results about worldsheet instantons in Type IIA Calabi-Yau
compactifications.
