Multi epoch spectroscopic variability of the B supergiant HD75149

Massive stars continuously enrich the surrounding interstellar medium by supplying it with stellar material driven by their powerful winds. B supergiant stars (BSGs) in particular are a type of massive star characterized by strong winds and notable photometric and spectroscopic variability. We aim to conduct a pilot study of the optical spectroscopic variability of the BSG HD75149 between 2004 and 2025. Its extended temporal baseline and pronounced variability amplitude make it particularly well suited for investigating the physical origin of the observed short-term variability within a consistent hydrodynamical and radiative-transfer framework. We analyzed 25 nightly averaged optical spectra obtained with different instruments and telescopes, some of them with observations over several consecutive days. We measured the radial velocities (RVs) and equivalent widths (EWs) of 17 spectral lines (H, HeI, SiIII, NII, MgII, CII). We modeled the Halpha emission, absorption, and P-Cygni profiles using the ISOSCELES grid and the delta-slow hydrodynamic regime. Halpha shows variability in intervals of a few days, including P-Cygni changes, while metal lines show small RV amplitudes, consistent with pulsating oscillations. The largest variation in the mass-loss rate corresponds to an increase of a factor of 1.8 within four days. In contrast, the terminal velocity remains barely affected during the same time interval. The pronounced variation observed in hydrogen lines, in contrast with the variability of other lines, suggests that it is due to mass-loss rate episodes driven by a slow wind occurring on a timescale comparable to photometric variations. We found no evidence of a close binary companion in the sample used, but we cannot completely exclude the possibility of a wide or low-inclination companion.