Graviton as a pair of collinear gauge bosons

We show that the mixed gravitational/gauge superstring amplitudes describing decays of massless closed strings – gravitons or dilatons – into a number of gauge bosons, can be written at the tree (disk) level as linear combinations of pure open string amplitudes in which the graviton (or dilaton) is replaced by a pair of collinear gauge bosons. Each of the constituent gauge bosons carry exactly one half of the original closed string momentum, while their ±1 helicities add up to ±2 for the graviton or to 0 for the dilaton.