  We study supervertices in six dimensional (2,0) supergravity theories, and
derive supersymmetry non-renormalization conditions on the 4- and 6-derivative
four-point couplings of tensor multiplets. As an application, we obtain exact
non-perturbative results of such effective couplings in type IIB string theory
compactified on K3 surface, extending previous work on type II/heterotic
duality. The weak coupling limit thereof, in particular, gives certain
integrated four-point functions of half-BPS operators in the nonlinear sigma
model on K3 surface, that depend nontrivially on the moduli, and capture
worldsheet instanton contributions.
