  We use insights from superstring field theory to prove the subleading soft
graviton theorem for tree amplitudes of (compactified) heterotic and type II
string theories for arbitrary number of finite energy NS (NSNS) sector external
states but only one soft graviton. We also prove the leading soft graviton
theorem in these theories for arbitrary number of external soft gravitons. In
our analysis there is no restriction on the mass and spin of the finite energy
external states. This method can also be used to give a proof of the subleading
soft graviton theorem for tree amplitudes in quantum field theories coupled to
gravity with generic interactions. We discuss the technical issue involved in
extending this analysis to loop amplitudes of superstring theory including
Ramond sector external states, and its possible resolution.
