Résumé
Euclidean D-branes wrapped on non-trivial cycles of a Calabi-Yau threefold are known to affect the metric on the hypermultiplet moduli space determining the effective action of type II strings on such manifolds. After reviewing the existing results on the instanton corrected metric following from a combination of constraints imposed by supersymmetry and dualities, I'll show how the D-instanton effects can be computed by a direct worldsheet approach. This required solving several conceptual and technical problems, which also paved the way for extension of this approach to compactifications with lower supersymmetry. The talk is based on joint work with A. Sen and B. Stefanski.