BPS states and supersymmetric index in N = 2 type I string vacua Original Research Article

We study the moduli dependence of a class of couplings in K3 × T2 compactifications of type I string theory, for which one-loop amplitudes can be written in terms of an N = 2 supersymmetric index. This index is determined for generic models as a function of the BPS spectrum. As an application we compute the one-loop moduli dependence of the FgW2g couplings, where W is the N = 2 gravitational superfield, for type I compactifications based on the Gimon-Johnson K3 orientifolds, showing explicitly their dependence on the aforementioned index.