Effective field theory from quantized strings

We present a covariant definition of the effective action Γ 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 Γ is reduced to that of establishing the effective action for a generalized σ-model on a curved two-dimensional background and subsequent integration over two-metrics. The low-energy approximation for Γ is computed.