A machine-learning photometric classifier for massive stars in nearby galaxies - I. The method Grigoris Maravelias, Alceste Z. Bonanos, Frank Tramper, Stephan de Wit, Ming Yang and Paolo Bonfini A&A, 666 (2022) A122 Published online: 14 October 2022 DOI: 10.1051/0004-6361/202141397