Testing the α'{}³ term in the non-abelian open superstring effective action

Recently, a proposal for the full non-abelian open superstring effective action through ${\cal O}(\alpha'{}^3)$ has been formulated in {\tt hep-th/0108169}. We test this result by calculating the spectrum in the presence of constant magnetic background fields and by comparing the result to string theoretic predictions. The agreement is perfect. Other proposals for the superstring effective action through this order do not reproduce the spectrum correctly.