ITP–UU–02/43\
SPIN–02/25\
hep-th/0208145

[**Instantons in the Double-Tensor Multiplet**]{}\
Ulrich Theis and Stefan Vandoren\
[ *Spinoza Instituut, Universiteit Utrecht\
Postbus 80.195, 3508 TD Utrecht, The Netherlands\
U.Theis, S.Vandoren@phys.uu.nl*]{}

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**Abstract**

The double-tensor multiplet naturally appears in type IIB superstring compactifications on Calabi-Yau threefolds, and is dual to the universal hypermultiplet. We revisit the calculation of instanton corrections to the low-energy effective action, in the supergravity approximation. We derive a Bogomol’nyi bound for the double-tensor multiplet and find new instanton solutions saturating the bound. They are characterized by the topological charges and the asymptotic values of the scalar fields in the double-tensor multiplet.

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Introduction
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Instanton effects in string and M-theory are still relatively poorly understood. This is due to the lack of a conventional instanton calculus as we know it from (supersymmetric) field theory. A well-known open problem is to determine the instanton corrections to the hypermultiplet moduli space of type II superstrings or M-theory compactified on a Calabi-Yau (CY) threefold down to four or five dimensions. Supersymmetry requires the hypermultiplet moduli space ${\cal M}_H$ to be quaternion-Kähler [@BW]. The four- (or five-) dimensional dilaton lives in a multiplet which can be dualized into the universal hypermultiplet. Hence, ${\cal M}_H$ 