High Five from ASTEP

Context. We present the analysis of five long-period TESS Objects of Interest (TOIs), all orbiting Sun-like stars, with orbital periods exceeding one month. Initially identified by the Transiting Exoplanet Survey Satellite (TESS), we extensively monitored these targets with the Antarctic Search for Transiting Exoplanets (ASTEP), supported by other facilities in the TESS Follow-up Observing Program (TFOP) network. Aims. These targets occupy a relatively underexplored region of the period-radius parameter space, offering valuable primordial probes for planetary formation and migration as warm planets better maintain their evolutionary fingerprints. Methods. To characterise these systems, we leveraged high-resolution speckle imaging to search for nearby stellar companions, and refine stellar parameters using both reconnaissance spectroscopy and spectral energy distribution (SED) fitting. We combined TESS photometry with high-precision ground-based observations from ASTEP, and when available, included additional photometry and radial velocity data. We applied statistical validation to assess the planetary nature of each candidate and used allesfitter to jointly model the photometric and spectroscopic datasets. Results. We validate the planetary nature of three TOIs, including the two warm Saturns TOI-4507 b (8.2 R⊕, 104 d) and TOI-3457 b (10.0 R⊕, 32.6 d), as well as the warm sub-Neptune TOI-707 b (2.4 R⊕, 52.8 d). The remaining two candidates are most consistent with eclipsing binaries, namely TOI-2404 and TOI-4404. Conclusions. These results help populate the sparse regime of warm planets, which serve as key tracers of planetary evolution, and demonstrate ASTEP’s effectiveness as a ground-based follow-up instrument for long-period systems.