OPD residuals as a function of magnitude for 10 μm rms atmospheric OPD, for 15 mas rms residual tip-tilt variations, and for the low vibration level (150 nm rms vibration OPD). Top: disturbance model is computed from a POL sequence acquired on the star itself (it is simply a zoom-in of Fig. 7, middle panel). Bottom: the model is computed observing a bright object (K = 7). The dashed lines correspond to the specification for the fringe tracker of GRAVITY which is to stabilize the fringes below 350 nm rms on a reference star of magnitude K = 10.