Putkuri, C., Gamen, R., Morrell, N., I, Maiz Apellaniz, J., Arias, J., I, Simon-Diaz, S., Ferrero, G. A., Rodriguez, C. N., Sota, A., Benvenuto, O. G., Barba, R. H.2024-11-272024-11-272023/09/110035-8711http://dx.doi.org/10.1093/mnras/stad2657https://publicacionesabiertas.userena.cl/handle/123456789/100We present the first spectroscopic orbit of the O-type double-lined star HD 168112 A,B. We analyse 101 high-resolution optical spectra identifying the absorption lines of both components. The orbital solution presents a relatively long period, P = 513.52 +/- 0.01 d, and a high eccentricity, e = 0.743 +/- 0.005. The binary system consists of two very similar stars of minimum masses of similar to 25 M-circle dot, effective temperatures of similar to 40 000 K, and surface gravities of similar to 3.7 dex. The system has a minimum semimajor axis a sin i similar to 1000 R-circle dot. We confirm that the A and B visual components identified via interferometry do correspond to the spectroscopic ones. We also analyse the underlying stellar groups using Gaia DR3 data and ground-based spectroscopy as part of the Villafranca project, determining that NGC 6604 is at a distance of 1942(-36)(+38) pc and giving spectral classifications for 23 massive stellar systems in Villafranca O-035 and the surrounding Ser OB2 association, for which we provide the most complete census of massive stars to date.ATMOSPHERIC NLTE-MODELS, B-TYPE STARS, FUNDAMENTAL PARAMETERS, ROTATIONAL VELOCITIES, RADIO-EMISSION, BLUE STARS, EVOLUTION, STELLAR, CLASSIFICATION, MULTIPLICITYArticle