A Note on Soft Factors for Closed String Scattering
  In this note, it is shown that closed string graviton scattering amplitudes obey the same subleading soft limit as field theory graviton scattering amplitudes. The result is derived using a combination of recent results and methods including the subleading soft expansion for type I open string gluon disk scattering amplitudes, the single value projection for multiple $\zeta$ values, and the KLT relations in field and string theory. 
Burkhard U. W. Schwab
hep-th