Heterotic String Field Theory with Manifest Spacetime Supersymmetry

Using the hybrid formalism with manifest $N=1$ $d=4$ spacetime supersymmetry, we construct the quadratic term in the heterotic superstring field theory action. As in open superstring field theory using the hybrid formalism, the heterotic string field theory action is constructed with three string fields and the massless sector describes $N=1$ $d=10$ supergravity in terms of $N=1$ $d=4$ superfields.