Variations on a theme: the puzzling behaviour of Schulte 12

One of the first massive stars detected in X-rays, Schulte 12 has remained a puzzle in several aspects. In particular, its extreme brightness both in the visible and X-ray ranges is intriguing. Thanks to Swift and XMM-Newton observations covering ~ 5000 d, we report the discovery of a regular 108 d modulation in X-ray flux of unknown origin. The minimum in the high-energy flux appears due to a combination of increased absorption and decreased intrinsic emission. We examined in parallel the data from a dedicated spectroscopic and photometric monitoring in the visible and near-IR domains, complemented by archives. While a similar variation timescale is found in those data, they do not exhibit the strict regular clock found at high energies. Changes in line profiles cannot be related to binarity but rather correspond to non- radial pulsations. Considering the substantial revision of the distance of Schulte 12 from the second Gaia data release, the presence of such oscillations agrees well with the evolutionary status of Schulte 12, as it lies in an instability region of the HR diagram. Based on X-ray observations collected with ASCA, Suzaku, Chandra and especially Swift and XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). Optical photometry from NSVS, Integral-OMC, and ASAS-SN are also used, as well as optical spectroscopy from the Hermes and Carmenes instruments. Hermes is installed on the Mercator Telescope, which is operated on the island of La Palma by the Flemish Community, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.