  We compare data of antineutron and antiproton annihilation cross sections on
different targets at very low energies. After subtracting Coulomb effects, we
observe that the ratio between the antineutron proton and antiproton proton
annihilation cross sections is an oscillating function of the energy at momenta
smaller 300 MeV/c. This nontrivial behavior is confirmed by the analysis of the
relative number of antiproton-neutron and antiproton-proton annihilations in
nuclei. We show that a part of the strong shadowing phenomena in
antiproton-nucleus annihilations can be explained in terms of this oscillation,
while a part requires different explainations.
