  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.
