Effect of the unitary mixing scalar--vector

We consider the procedure of dressing of scalar and vector particles when there exists the off--diagonal loop connecting vector and scalar propagators. Instead of single Dyson equations for scalar and vector, we have in this case a system of three equations for coupled full propagators. Using the $\pi-a_1$ system as an example, we discuss the physical meaning of this effect and the renormalization procedure for coupled propagators. The considered effect of the unitary S~--~V mixing may exits also in the "Higgs boson~--~Z~(~W~)" system for extended electroweak models.