  We analyze the form factors parametrizing the $B_c \to J/\psi, \eta_c$ matrix
elements of the operators in a generalized low-energy $b \to c$ semileptonic
Hamiltonian. We consider an expansion in nonrelativistic QCD, classifying the
heavy quark spin symmetry breaking terms and expressing the form factors in
terms of universal functions in a selected kinematical range. Using as an input
the lattice QCD results for the $B_c \to J/\psi$ matrix element of the SM
operator, we obtain information on other form factors. The extrapolation to the
full kinematical range is also presented.
