SUPERSPACE EFFECTIVE ACTIONS FOR 4D COMPACTIFICATIONS OF HETEROTIC AND TYPE-II SUPERSTRINGS

Two-dimensional sigma models are defined for the new manifestly spacet ime supersymmetric description of four-dimensional compactified supers trings. The resulting target-superspace effective action is constraine d by the way the spacetime dilaton couples to the worldsheet curvature : For the heterotic superstring, the worldsheet curvature couples to t he real part of a chiral multiplet, and for Type II it couples to the real part of the sum of a vector multiplet and a tensor hypermultiplet . For the Type II superstring, this contradicts the standard folklore that only a hypermultiplet counts string loops, explains the peculiar dilaton coupling of Ramond-Ramond fields, and allows the effective act ion to be easily written in N = 2 4D superspace, It also implies that vector multiplet interactions get no quantum corrections, while hyperm ultiplet interactions can only get corrections if minor symmetry is no n-perturbatively broken.