Variational problem for the Frenkel and the Bargmann-Michel-Telegdi   (BMT) equations
  We propose Lagrangian formulation for the particle with value of spin fixed within the classical theory. The Lagrangian turns out to be invariant under non-abelian group of local symmetries. As the gauge-invariant variables for description of spin we can take either the Frenkel tensor or the BMT vector. Fixation of spin within the classical theory implies $O(\hbar)$-corrections to the corresponding equations of motion. 
A. A. Deriglazov
hep-th math-ph math.MP quant-ph