| Model independent properties of the matrix element and polarization observables are derived for electron positron annihilation in different channels involving spin one particles. The general spin structure depends on the reaction mechanism, here assumed one photon exchange. The expression of the observables are derived as functions of the hadron electromagnetic form factors, and are given here, as an example, for the reaction $e^++e^-\\to a_1+\\pi$. Hadron form factors should obey specific analytical properties, in space and time-like regions. Possible parametrizations and experimental constraints are illustrated.
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