TOI-5126: a hot super-Neptune and warm Neptune pair discovered by TESS and CHEOPS

We present the confirmation of a hot super-Neptune with an exterior Neptune companion orbiting a bright (V = 10.1 mag) F-dwarf identified by the Transiting Exoplanet Survey Satellite (TESS). The two planets, observed in sectors 45, 46, and 48 of the TESS extended mission, are 4.74(-0.14)(+0.16) and 3.86(-0.16)(+0.17) R-circle plus with 5.4588385(-0.0000072)(+0.0000070) and 17.8999(-0.0013)(+0.0018 )d orbital periods, respectively. We also obtained precise space-based photometric follow-up of the system with ESA's CHaracterising ExOplanets Satellite to constrain the radius and ephemeris of TOI-5126 b. TOI-5126 b is located in the 'hot Neptune Desert' and is an ideal candidate for follow-up transmission spectroscopy due to its high-predicted equilibrium temperature (T-eq = 1442(-40)(+46) K) implying a cloud-free atmosphere. TOI-5126 c is a warm Neptune (T-eq = 971(-27)(+31) K) also suitable for follow-up. Tentative transit timing variations have also been identified in analysis, suggesting the presence of at least one additional planet, however this signal may be caused by spot-crossing events, necessitating further precise photometric follow-up to confirm these signals.