  Some instanton corrections to the universal hypermultiplet moduli space
metric of the type-IIA string theory compactified on a Calabi-Yau threefold
arise due to multiple wrapping of BPS membranes and fivebranes around certain
cycles of Calabi-Yau. The classical universal hypermultipet metric is locally
equivalent to the Bergmann metric of the symmetric quaternionic space
SU(2,1)/U(2), whereas its generic quaternionic deformations are governed by the
integrable SU(infinity) Toda equation. We calculate the exact
(non-perturbative) UH metrics in the special cases of (i) the D-instantons (the
wrapped D2-branes) in the absence of fivebranes, and (ii) the fivebrane
instantons with vanishing charges, in the absence of D-instantons. The
solutions of the first type preserve the U(1)xU(1) classical symmetry, while
they can be interpreted as the gravitational dressing of the hyper-K"ahler
D-instanton solutions. The second type solution preserves the non-abelian SU(2)
classical symmetry, while it can be interpreted as a gradient flow in the
universal hypermultiplet moduli space.
