6160 20230123211436.0 10.1051/0004-6361/202244148 DOI SCART-2023-0085 Abdul-Masih, Michael Constraining the overcontact phase in massive binary evolution. II. Period stability of known O+O overcontact systems 2022 Context. Given that mergers are often invoked to explain many exotic phenomena in massive star evolution, understanding the evolutionary phase directly preceding a merger, the overcontact phase, is of crucial importance. Despite this, large uncertainties exist in our understanding of the evolution of massive overcontact binaries. Aims: We aim to provide robust observational constraints on the future dynamical evolution of massive overcontact systems by measuring the rate at which the periods change for a sample of six such objects. Furthermore, we aim to investigate whether the periods of unequal-mass systems show higher rates of change than their equal mass counterparts, as theoretical models predict. Methods: Using archival photometric data from various ground- and space-based missions covering up to ∼40 years, we measure the periods of each system over several smaller time spans. We then fit a linear regression through the measured periods to determine the rate at which the period is changing over the entire data set. Results: We find that all of the stars in our sample have very small period changes and that there does not seem to be a correlation with the mass ratio. This implies that the orbital periods for these systems are stable on the nuclear timescale, and that the unequal-mass systems may not equalize as expected. Conclusions: When comparing our results with population synthesis distributions, we find large discrepancies between the expected mass ratios and period stabilities. We find that these discrepancies can be mitigated to a degree by removing systems with shorter initial periods, suggesting that the observed sample of overcontact systems may originate from binary systems with longer initial orbital periods. NO binaries: close stars: massive stars: evolution techniques: photometric Astrophysics - Solar and Stellar Astrophysics Escorza, Ana Menon, Athira Mahy, Laurent Marchant, Pablo Astronomy & Astrophysics 666 2022 A18 11 laurent.mahy@ksb-orb.be 686474 http://publi2-as.oma.be/record/6160/files/Abdul-Masih_2022_aa44148-22.pdf 3401 http://publi2-as.oma.be/record/6160/files/Abdul-Masih_2022_aa44148-22.gif?subformat=icon icon 8367 http://publi2-as.oma.be/record/6160/files/Abdul-Masih_2022_aa44148-22.jpg?subformat=icon-180 icon-180 published in REFERD