Group-delay tracking and visibility fluctuations in long-baseline stellar interferometry

We investigate the limitations of group-delay fringe tracking for stellar i nterferometry in the presence of realistic fluctuations of the fringe visib ility amplitude. These fluctuations can be considered as introducing an add itional source of noise beyond those attributable to counting statistics an d detector readout, and lead to a significant increase in the probability o f tracking failure compared with previous studies. We show that even for re latively bright targets, the probability of tracking loss can remain very h igh (> 0.5) unless the seeing conditions are favourable.