  We report Electron Paramagnetic Resonance measurements on single crystalline
and powder samples of Nd_0.5Ca_0.5MnO_3 across the charge-ordering transition
at T_co=240 K down to the antiferromagnetic ordering transition at T_N = 140 K.
The changes in the linewidth, g-factor and intensity as functions of
temperature are studied to understand the nature of spin-dynamics in the
system. We explain the observed large decrease in the linewidth from T_N to
T_co in terms of motional narrowing caused by the hopping of the Jahn-Teller
polarons yielding an activation energy of E_a = 0.13 eV. Similar analysis of
data on Pr_0.6Ca_0.4MnO_3 published earlier gives E_a=0.2 eV. Below T_co, the
g-value increases continuously suggesting a gradual strengthening of the
orbital ordering. We give a qualitative explanation of the temperature
dependence and of the maximum in the asymmetry ratio A/B observed at T_co in
single crystal spectra which also supports the themodel of motional narrowing.
