Azimuthal brightness modulation reveals hidden rings in CI Tau

Context. Protoplanetary disks often host substructures such as rings and gaps, which trace key processes in planet formation and dust evolution. However, narrow rings may remain unresolved due to limited observational resolution, hiding critical information about early planetesimal formation and the amount of dust present. Aims. We applied the azimuthal brightness modulation method, based on the modulation produced by multiple unresolved optically thick rings embedded in an optically thin background, to multiwavelength ALMA observations of CI Tau to identify unresolved rings and constrain their geometry and optical depth. Methods. We analysed CI Tau archival ALMA continuum observations in bands 3, 6, and 7, and extracted azimuthal brightness profiles along narrow annuli and compared them with forward-modelled synthetic observations of inclined discs containing unresolved rings. Results. We detect the azimuthal signature at ∼22 au in all three bands, consistent with unresolved, optically thick rings embedded in a lower optical depth background. Multiwavelength modelling constrains the rings’ geometry and optical depth, consistent with the conditions expected for streaming instability and early planetesimal formation. Conclusions. Our results demonstrate the applicability of this azimuthal signature technique to real discs, reveal fine-scale dust substructures in CI Tau, and illustrate a new method of studying the early stages of planet formation below the nominal resolution limit.