  A modified fluctuation-dissipation-theorem (MFDT) for a non-equilibrium
steady state (NESS) is experimentally checked by studying the position
fluctuations of a colloidal particle whose motion is confined in a toroidal
optical trap. The NESS is generated by means of a rotating laser beam which
exerts on the particle a sinusoidal conservative force plus a constant
non-conservative one. The MFDT is shown to be perfectly verified by the
experimental data. It can be interpreted as an equilibrium-like
fluctuation-dissipation relation in the Lagrangian frame of the mean local
velocity of the particle.
