  We construct the derivative corrections to the four-point vertices in the
abelian open string effective action to all orders in alpha'. The result is
based on the structure of the string four-point function. Supersymmnetry of
these vertices is guaranteed by the supersymmetry of the F^4 term in the
effective action. By this construction we establish the existence of an
infinite number of supersymmetry invariants, the number of invariants at order
alpha'^n grows linearly with n.
