We evaluate the two-photon box contribution to heavy fermion production in electron positron annihilation, that provides ({cal O}(alpha^2)) electromagnetic corrections to the Born cross section. The study of its non-relativistic expansion, relevant at energies close to the threshold of production, is also performed. We also verify that the threshold expansion of the one-loop integrals correctly reproduces our results, thus extending the applicability of this technique to heavy fermion production diagrams.
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