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The derivation of the low energy theorem for threshold photoproduction of neutral pions on a nucleon is reconsidered. Instead of considering explicitly the Born terms, as done in earlier derivations, a more general class of diagrams is separated from the total amplitude. Gauge invariance is imposed on the operator level and the partial conservation of the axial-vector current constraint is incorporated. Besides recovering the older results in this approach, relations for the off-shell behavior of the strong and electromagnetic vertices are derived.

Off-shell form factors and low energy theorems for pion photoproduction