Q-instantons

We construct the half-supersymmetric instanton solutions that are electric-magnetically dual to the recently discussed half-supersymmetric Q7-branes. We call these instantons "Q-instantons". Whereas the D-instanton is most conveniently described using the RR axion chi and the dilaton phi, the Q-instanton is most conveniently described using a different set of fields (chi', T), where. chi' is an axionic scalar. The real part of the Q-instanton on-shell action is a function of T and the imaginary part is linear in chi'. Discrete shifts of the axion chi' correspond to PSL(2, Z) transformations that are of finite order. These are e. g. pure S-duality transformations relating weak and strongly coupled regimes. We argue that near each orbifold point of the quantum axion-dilaton moduli space {tau vertical bar tau is an element of(PSL(2,R))(SO(2)xPSL(2,L))} the higher order R-4 terms in the string effective action contain contributions from an infinite sum of single multiply-charged instantons with the Q-instantons corresponding to the orbifold points tau = i,rho.