  We calculate the fragmentation function for a light quark to decay into a
lepton pair to leading order in the QCD coupling constant. In the formal
definition of the fragmentation function, a QED phase must be included in the
eikonal factor to guarantee QED gauge invariance. We find that the longitudinal
polarization fraction is a decreasing function of the factorization scale, in
accord with the intuitive expectation that the virtual photon should behave
more and more like a real photon as the transverse momomentum of the
fragmenting quark increases.
