Nested, asymmetric H-He circumstellar shells in the Type Icn/Ibn SN 2024abvb

Context. Interacting transients probe mass loss in the final stages of stellar evolution; however, the geometry and timing of multi-episode mass loss remain poorly constrained. SN 2024abvb is a nearby interacting event with transitional Ibn/Icn spectroscopic properties and multi-epoch polarimetry, offering a rare opportunity to study structured circumstellar material (CSM). Aims. We aim to characterise the kinematics, composition, and geometry of the CSM around SN 2024abvb and to identify plausible progenitor and ejection scenarios that can produce the observed spectro-polarimetric evolution. Methods. We present high-resolution (VLT/UVES and VLT/X-Shooter) optical/NIR spectroscopy across several epochs, complemented by broadband polarimetry and spectropolarimetry (VLT/FORS2 and NOT/ALFOSC). Line identifications, velocity decompositions, and polarimetric time series were used to trace multiple kinematic components and changes in scattering geometry. Results. The high-resolution spectra reveal multiple narrow CSM components composed of He, C, and O with absorption minima at similar to 150 - 400 km s(-1) and additional faster material up to similar to 2000 km s(-1). Low-velocity Balmer absorptions are present, indicating distant H-rich material, a first in Type Ibn/Icn supernovae. Polarimetry shows a marked evolution (P similar to 1% near the peak, less than or similar to 0.5% after similar to 1 week, rising to similar to 1.5% after similar to 20 d with similar to 50 degrees position-angle rotation and to similar to 4% after similar to 30 d, stronger in the blue), implying a time-variable, wavelength-dependent scattering and obscuration component. The combination of kinematics and polarimetric behaviour is consistent with multiple, concentric toroidal shells with differing orientations and partial dust content.