Relativistic Instant--Form Approach To The Structure Of Two-Body Composite Systems. Nonzero Spin

The relativistic approach to electroweak properties of two-particle composite systems developed in Ref. [A.F.Krutov, V.E.Troitsky, Phys.Rev.C. 65 (2002) 045501] is generalized here to the case of nonzero spin. In developed technique the parametrization of matrix elements of electroweak current operators in terms of form factors is a realization of the Wigner--Eckart theorem on the Poincar\'e group and form factors are reduced matrix elements. The $\rho$ meson charge form factor is calculated as an example.