  For the study of deep inelastic scattering at HERA, the use of Jaquet-Blondel
variables is advantageous in several respects. We calculate the complete
leptonic ${\cal O}(\alpha)$ QED corrections for the reaction $ep \to eX$ in
these variables. All but one phase space integrations are performed
analytically. After exponentiation of soft photon corrections, an accuracy of
at least 1% is matched. Numerical results are presented and compared to
estimates based on the leading logarithmic approximation. Equation (17) is
corrected in accordance with equation (7.44) of DESY 94--115. The misprint did
not influence the numerics.
