  Using local gauge invariance in the form of the Ward-Takahashi identity and
the fact that properly constructed current operators must be free of kinematic
singularities, it is shown that the magnetic moment $\mu$ and the quadrupole
moment $Q$ of an elementary spin-1 particle with mass $m$ and charge $e$ are
related by $2 m\mu + m^2 Q = e$, thus constraining the normalizations of the
Sachs form factors. This relation holds true as a matter of course at the tree
level in the standard model, but we prove it remains true in general for
dressed spin-1 states derived from elementary fields. General expressions for
spin-1 propagators and currents with arbitrary hadronic dressing are given
showing the result to be independent of any dressing effect or model approach.
