  We derive a massive double copy construction within string theory. To this
end, we use massive vectors of the open string spectrum that appear in
compactifications to four dimensions and construct massive spin-2 tensors as
closed string states, thereby mimicking the structure of the massless graviton.
We then compute three-point amplitudes for the scattering of massless and
massive spin-2 closed string states and reveal the double copy structure of the
latter. With these results being finite in the string scale, we are further
able to reproduce the cubic Lagrangian of ghost-free bimetric theory around
flat spacetime for bulk massive spin-2 states originating in products of
vectors of extended brane supersymmetry.
