  We calculate the synchrotron and inverse-Compton emissions from pairs formed
in GRB afterglows from high-energy photons (above 100 MeV), assuming a
power-law photon spectrum C_nu ~ nu^{-2} and considering only the pairs
generated from primary high-energy photons. The essential properties of these
pairs (number, minimal energy, cooling energy, distribution with energy) and of
their emission (peak flux, spectral breaks, spectral slope) are set by the
observables GeV fluence Phi (t) = Ft and spectrum, and by the Lorentz factor
Gamma and magnetic field B of the source of high-energy photons, at
observer-time t. Optical and X-ray pseudo--light-curves F_nu (Gamma) are
calculated for given B; proper synchrotron self-Compton light-curves are
calculated by setting the dynamics Gamma(t) of the high-energy photons source
to be that of a decelerating, relativistic shock. It is found that the emission
from pairs can accommodate the flux and decays of the optical flashes measured
during the prompt (GRB) phase and of the faster-decaying X-ray plateaus
observed during the delayed (afterglow) phase. The brightest pair optical
emission is obtained for 100 < Gamma < 500, and depends mostly on the GeV
fluence, being independent of the source redshift. Emission from pairs formed
during the GRB phase offers an alternate explanation to reverse-shock optical
flashes. These two models may be distinguished based on their corresponding
flux decay index--spectral slope relations, different correlations with the LAT
fluence, or through modeling of the afterglow multiwavelength data.
