2025MNRAS.537.2334V

In recent years, intensity interferometry has been successfully applied to the Imaging Atmospheric Cherenkov Telescopes, H.E.S.S. , MAGIC, and VERITAS. All three telescope systems have proven the feasibility and capability of this method. After our first campaign in 2022, when two of the H.E.S.S. telescopes in Namibia were equipped with our external set-up and the angular diameter of two stars was measured, our set-up was upgraded for a second campaign in 2023, where the goal is to perform simultaneous two-colour measurements. The second campaign not only involves a third equipped telescope, but also each mechanical set-up now includes two interference filters at two different wavelengths (375 and 470 nm) with a broader bandwidth of 10 nm. This enables having simultaneous two-colour measurements, which yield information about the star's physical size at different wavelengths. This is the first time that simultaneous dual-waveband intensity interferometry measurements have been performed. We report the angular diameter results of four stars, Mimosa ($\beta$ Cru), Eta Centauri ($\eta$ Cen), Nunki (σ Sgr), and Dschubba (δ Sco), where the effects of limb darkening are also taken into account.