The subtraction constant in π± electroproduction

A method is proposed for dealing with the subtraction constant required in fixed-t dispersion relations for π^± electroproduction. The method requires a knowledge of the t-channel cut contribution (evaluated at the subtraction points s₀), which can be determined at k² = 0 using a gauge invariance sum rule. Estimates by this and other methods show that it is small (compared with the pion pole contribution) for ?t ? 1 GeV², and in particular it is irrelevant to determinations of the pion form factor using high-energy forward π⁺ data.