2026A&A...705A..43M

Context. Massive contact binaries (CBs) are crucial objects for understanding close binary evolution and stellar mergers. Study of these objects has been hampered by a scarcity of observed systems, particularly of B-type systems which are expected to dominate this class.
Aims. We bridge this observational gap by mining a large sample of massive CB candidates from the OGLE-IV database, potentially increasing their current numbers in the Magellanic Clouds by an order of magnitude.
Methods. Using main-sequence color-magnitude limits, an observationally informed period-luminosity-color relation for CBs, and a high morph parameter cut (c ≥ 0.7), we empirically identified a subsample of 68 O and B-type binaries with periods P < 3 days, that exhibit smooth, sinusoidal light curves with nearly equal eclipse depths. To mine our bona fide sample of CB candidates among these, we used theoretical color-magnitude and orbital period distributions based on a vast grid of MESA binary models. We also computed synthetic light curves using PHOEBE corresponding to the contact and near-contact phases of a MESA model.
Results. Our bona fide candidate CB sample consists of 37 systems (9 in the SMC and 28 in the LMC), that fulfill the theoretical predictions for massive CBs. The bona fide sample, which predominantly consists of B-type binaries with periods of P ≈ 0.6 - 1 day, closely agrees with our predicted population count. As our binary models predict mass equalization followed by temperature equalization during nuclear-timescale contact, a substantial fraction of these bona fide CB candidates may have mass ratios of q ≈ 1.
Conclusions. Our work significantly expands the observational sample of B-type candidate massive CBs. Furthermore, our synthetic light curves show a degeneracy between contact and near-contact binary light curves, indicating the possibility of misidentifications between these configurations when characterized based on light curves alone. Spectroscopic follow-up is necessary to test our predictions, particularly for the mass ratios of these CB candidates.