  In many GRBs, afterglows have been observed for months or even years. It
deserves noting that at such late stages, the remnants should have entered the
deep Newtonian phase, during which the majority of shock-accelerated electrons
will no longer be highly relativistic. However, a small portion of electrons
are still ultra-relativistic and capable of emitting synchrotron radiation.
Under the assumption that the electrons obey a power-law distribution according
to their kinetic energy (not simply the Lorentz factor), we calculate optical
afterglows from both isotropic fireballs and beamed ejecta, paying special
attention to the late stages. In the beamed cases, it is found that the light
curves are universally characterized by a flattening during the deep Newtonian
phase. Implication of our results on orphan afterglows is also addressed.
