Effective field theory from quantized strings

We present a covariant definition of the effective action GAMMA for fields corresponding to different string excitations (scalar, metric, antisymmetric tensor, ...) in terms of a string path integral with ''external sources''. The problem of deriving the gravitational part of GAMMA is reduced to that of establishing the effective action for a generalized sigma-model on a curved two-dimensional background and subsequent integration over two-metrics. The low-energy approximation for GAMMA is computed. (orig.)