Radiation of light fermions in heavy fermion production

The rate for the production of a pair of massive fermions in e⁺e^- annihilation plus real or virtual radiation of a pair of massless fermions is calculated analytically. The contributions for real and virtual radiation are displayed separately. The asymptotic behaviour close to threshold and for high energies is given in a compact form. These approximations provide arguments for the appropriate choice of the scale in the O(a) result, such that no large logarithms remain in the final answer.