Exact thresholds and instanton effects in D=3 string theories

Three-dimensional string theories with 16 supersymmetries are believed to p ossess a non-perturbative U-duality symmetry SO(8, 24, Z). By covariantizin g the heterotic one-loop amplitude under U-duality, we propose an exact exp ression for the (partial derivative phi)(4) amplitude, that reproduces know n perturbative limits. The weak-coupling expansion in either of the heterot ic, type-II or type-I descriptions exhibits the well-known instanton effect s, plus new contributions peculiar to three-dimensional theories, including the Kaluza-Klein 5-brane, for which we extract the summation measure. This letter is a post-scriptum to hep-th/0001083.