MONOS: Multiplicity Of Northern O-type Spectroscopic systems

Context. Understanding massive star multiplicity is essential for constraining formation scenarios, binary evolution, and stellar feedback in galaxies. The MONOS (Multiplicity Of Northern O-type Spectroscopic systems) project aims to characterize O-type spectroscopic binaries in the northern hemisphere (δ > −20°), combining high-resolution spectroscopy and multi-epoch photometry. Aims. This study uses Gala DR3 and TESS epoch photometry to identify orbital variability in O-type stars and derive orbital solutions. Methods. We analyzed ten O-type binary systems with high-quality Gala and TESS photometry and available high-resolution spectra. We complemented our analysis with additional photometric data from the Hipparcos mission or the early-stage MUDEHaR survey. Periods were derived using three independent techniques and combined with radial velocity measurements to model each system using the PHOEBE code, yielding orbital and stellar parameters. Results. We present eight previously unpublished orbits – two with newly determined periods – and refine two others. In several cases, our periods match Gala values, though we highlight issues in automated determinations, such as half-period aliases. Among our notable discoveries, we point out the first known Oe+O spectroscopic binary (BD +61 487) and a system of overcontact O-type supergiants in an eccentric orbit where significant mass transfer has taken place (HD 169 727). The derived solutions are consistent with spectral classifications and theoretical expectations, including short periods (< 3 days), high mass ratios, and semidetached or overcontact configurations. Conclusions. These results expand the sample of O-type binaries with robust orbital characterization, especially in the short-period regime in which tidal effects and mass transfer are prominent. The combined use of Gala, TESS, and spectroscopy is shown to be effective, offering a scalable methodology applicable to southern hemisphere surveys. This work provides a foundation for future quantitative spectroscopic analyses and aids in achieving a comprehensive insight into the evolution of massive multiple systems.