  The elliptic-flow ratio of neutrons with respect to protons or light complex
particles in reactions of neutron-rich systems at relativistic energies is
proposed as an observable sensitive to the strength of the symmetry term in the
equation of state at supra-normal densities. The results obtained from the
existing FOPI/LAND data for 197Au + 197Au collisions at 400 MeV/nucleon in
comparison with the UrQMD model favor a moderately soft symmetry term with a
density dependence of the potential term proportional to rho/rho_0^gamma with
gamma = 0.9 +- 0.4.
