  The in-medium modifications on the $K^-/K^+$ ratio produced at GSI are
studied. Particular attention is paid to the properties of antikaons, which
determine the chemical potential and temperature at freeze-out conditions.
  Different approaches have been considered: non-interacting $K^-$, on-shell
self-energy and single-particle spectral density. We observe that the full
off-shell approach reproduces the "broad-band equilibration" advocated by Brown
being the spectral density crucial to explain an enhanced $K^-/K^+$ ratio.
