Fringe sensing noise plotted at percentile 0.9 (i.e., the noise is actually lower than the plotted curves for 90% of the occurrences) as a function of the target’s K magnitude (or of the stellar flux in photons per second at the entrance of 2GFT) in the 4T3-ABCD case, assuming a K0 III target and using the A0-G1-K0-I1 quadruplet of ATs at the VLTI. The fringe-tracking loop is operated at its maximum frequency as long as the fringe sensing noise per baseline remains  < 100 nm rms for 90% of the measurements on any individual baseline. The closed-loop repetition frequency is reduced to maintain this level of performance otherwise (this happens for K > 5 in the present case, as also shown in Fig. 10), until this level cannot be reached any more (beyond K = 7.5 in the present case). Note that the increase in sensing noise for bright targets is caused by the resolved stellar photosphere, which reduces the available coherent flux.