Abstract
The effect of an anomalous magnetic moment Δκ of the W boson on the photon structure is calculated. The result depends quadratically on the cutoff Λ. Comparison with data from DESY storage ring PETRA gives a limit ‖Δκ(Λ/MW)‖≲33. This bound is compared with the constraint from the anomalous magnetic moment of the muon. Also corrections to the ρ parameter are discussed.