We construct the most general non-extremal spherically symmetric instanton solution of a gravity-dilaton-axion system with $SL(2,R)$ symmetry, for arbitrary euclidean spacetime dimension $Dgeq 3$. A subclass of these solutions describe completely regular wormhole geometries, whose size is determined by an invariant combination of the $SL(2,R)$ charges. Our results can be applied to four-dimensional effective actions of type II strings compactified on a Calabi-Yau manifold, and in particular to the universal hypermultiplet coupled to gravity. We show that these models contain regular wormhole solutions, supported by regular dilaton and RR scalar fields of the universal hypermultiplet.
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