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 We present eight previously unpublished orbits – two with newly determined periods – and refine two others.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.