RR - Dilaton Interaction In a Type IIB Superstring

We analyze the interaction between the massless Ramond-Ramond states with a dilaton in a type II superstring.By constructing vertex operators for massless Ramond-Ramond states and computing their correlation functions with a dilaton we find the RamondRamond part of the superstring low-energy effective action.Those RR terms appear in the action without the standard dilatonic factor (string coupling constant) as has been shown earlier on the basis of space-time supersymmetry. The geometrical interpretation of this fact is presented.Namely we argue that the spin operators in the RR vertices effectively decrease the Euler character of the worldsheet by 1 unit.As a result,the dilaton term in the worldsheet action has the form: ∼ ∫ dzeΛF Λ̄, where Λ and Λ̄ are the (1, 0) and (0, 1) spin operators for the ghost and matter fields,F is the RR field strength contracted with γ-matrices. We also analyse the interaction of a dilaton with the massive RR states.