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I was reading Mandl and Shaw's book on quantum field theory and on page 184 I came across the sentence "the photon propagator always occurs coupled to conserved currents". This allows him to disregard the term proportional to $k^{\mu} k^{\nu}$ in the photon self energy $\Pi^{\mu \nu} (k)$. I understand this is true if the photon propagator in question is coupled with the external legs (which in the S matrix expansion would indeed be given by $j^{\mu} (x) = N[\overline{\psi}\gamma^{\mu} \psi]_x$). But in case it was coupled to internal lines how could it be coupled to conserved currents? It would just be coupled to two propagators like $S_F(p_1)$ and $S_F(p_2)$, with off-shell 4-momenta.

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