TOI-5126: A hot super-Neptune and warm Neptune pair discovered by \(\textit{TESS}\) and \(\textit{CHEOPS}\)

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 \(\textit{Transiting Exoplanet Survey Satellite}\) (\(\textit{TESS}\)). The two planets, observed in sectors 45, 46 and 48 of the \(\textit{TESS}\) extended mission, are \(4.74^{+0.16}_{-0.14}\) \(R_{\oplus}\) and \(3.86^{+0.17}_{-0.16}\) \(R_{\oplus}\) with \(5.4588385^{+0.0000070}_{-0.0000072}\) d and \(17.8999^{+0.0018}_{-0.0013}\) d orbital periods, respectively. We also obtained precise space based photometric follow-up of the system with ESAs \(\textit{CHaracterising ExOplanets Satellite}\) (\(\textit{CHEOPS}\)) 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 (\(T_{eq} = 1442^{+46}_{-40}\) K) implying a cloud-free atmosphere. TOI-5126 c is a warm Neptune (\(T_{eq}= 971^{+31}_{-27}\) K) also suitable for follow-up. Tentative transit timing variations (TTVs) 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.