The IACOB project - VI. On the elusive detection of massive O-type stars close to the ZAMS G. Holgado, S. Simón-Díaz, L. Haemmerlé, D. J. Lennon, R. H. Barbá, M. Cerviño, N. Castro, A. Herrero, G. Meynet and J. I. Arias A&A, 638 (2020) A157 Published online: 30 June 2020 DOI: 10.1051/0004-6361/202037699