Long-term multi-band photometric monitoring of Mrk 501 Axel Arbet-Engels, Dominik Baack, Matteo Balbo, Adrian Biland, Thomas Bretz, Jens Buss, Daniela Dorner, Laura Eisenberger, Dominik Elsaesser, Dorothee Hildebrand et al. (11 more) A&A, 655 (2021) A93 Published online: 25 November 2021 DOI: 10.1051/0004-6361/202141886