Long-term pulse period evolution of the ultra-luminous X-ray pulsar NGC 7793 P13 F. Fürst, D. J. Walton, M. Heida, M. Bachetti, C. Pinto, M. J. Middleton, M. Brightman, H. P. Earnshaw, D. Barret, A. C. Fabian et al. (7 more) A&A, 651 (2021) A75 Published online: 16 July 2021 DOI: 10.1051/0004-6361/202140625