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Long-period transiting planets are essential edge-cases for understanding exoplanet formation and evolution. Their detection is challenging due to the infrequent nature of their transits, making every observation of these objects very valuable. Our goal is to confirm and further characterise the long-period planetary candidate TOI-4409,b (initially identified by TESS) through additional observations using the Antarctica-based ASTEP telescope, supplemented with CHAT, OMES, LCOGT, and PEST light-curve data and radial-velocity (RV) measurements collected with the HARPS, FEROS, and PFS instruments. We jointly analysed the photometric data from eight TESS, four ASTEP, one CHAT, two OMES, four LCOGT, and one PEST light curve(s), and RV data from the HARPS, FEROS, and PFS instruments, covering observations from 2018 to 2024. TOI-4409,b is a bona fide puffy warm super-Neptune exoplanet with a planet radius of 7.44 ± 0.19,R_⊕, and a mass of 0.047 ± J ,(3σ upper limit <0.095,M_ J) orbiting a low-mass main-sequence star (effective temperature of 4928 ± 48, and a stellar radius of 0.720 ± 0.018, _⊙) with a period of 92.49179 ± 0.00010 days. The integration of various observational methods and consistent transit signals across multiple instruments confirms the planetary nature of TOI-4409,b. The combined transit, imaging, chromaticity, and RV evidence supports the confirmation of the planet. K R With a radius of 7.44 ± 0.19,R_⊕ and a low bulk density of 0.20 ± 0.07 -3 $, TOI-4409,b occupies a sparsely populated region of parameter space among warm, puffy super-Neptunes on long-period orbits. The detection of candidate transit timing variations with an ∼10-minute semi-amplitude hints at the presence of an additional body in the system. The favourable host-star brightness and extended transit duration ($∼7 hours) make TOI-4409,b a promising target for atmospheric characterisation with JWST and Ariel.
