2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/188451We find the D(-1) and D1-brane instanton contributions to the hypermultiplet moduli space of type IIB string compactifications on Calabi-Yau threefolds. These combine with known perturbative and worldsheet instanton corrections into a single modular invariant function that determines the hypermultiplet low-energy effective acction.5 pages, 1 figure; v2: minor changes, references added, title changed as requested by PRLHigh Energy Physics - Theorytext