The occurrence and impact of carbon-oxygen shell mergers in massive stars L. Roberti, M. Pignatari, H. E. Brinkman, S. K. Jeena, A. Sieverding, A. Falla, M. Limongi, A. Chieffi and M. Lugaro A&A, 698 (2025) A216 Published online: 17 June 2025 DOI: 10.1051/0004-6361/202554461