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We study the behavior of the scattering amplitudes of the bosonic string involving a soft massless state (graviton, dilaton and Kalb-Ramond antisymmetric tensor) and closed string tachyons or other closed string massless states. For a soft graviton we confirm the results, obtained in ref. 37 using just gauge invariance, up to terms of O (q¹) for external tachyons and up to terms of O (q⁰) for external massless closed string states. Furthermore, we also derive the behavior of the scattering amplitude when a dilaton or a Kalb-Ramond field becomes soft. These results are new and cannot, to our knowledge, be derived by using gauge invariance. It turns out, in the cases examined, that the soft amplitude for a dilaton or for an antisymmetric tensor is obtained by saturating the tensor, M μν, derived from gauge invariance for gravitons, with their respective polarization tensors. Thus extra terms that could have appeared in M μν in the case of a soft dilaton, in fact do not appear.
Vecchia et al. (2015) studied this question.