The GRAVITY fringe tracker S. Lacour, R. Dembet, R. Abuter, P. Fédou, G. Perrin, É. Choquet, O. Pfuhl, F. Eisenhauer, J. Woillez, F. Cassaing et al. (11 more) A&A, 624 (2019) A99 Published online: 18 April 2019 DOI: 10.1051/0004-6361/201834981