  The non-perturbative (D-instanton corrected) low-energy effective action of
the universal hypermultiplet in the type-IIA string theory compactified on a
Calabi-Yau threefold is calculated in 4d, N=2 supergravity. The action is given
by the quaternionic non-linear sigma-model whose metric is governed by the
Eisenstein series E_{3/2}. The U(1)xU(1) isometry and SL(2,Z) modular
invariance play the key role in our construction.
