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​R⊕​ and 3.86−0.16+0.17​R⊕​ with 5.4588385−0.0000072+0.0000070​ d and
17.8999−0.0013+0.0018​ d orbital periods, respectively. We also obtained
precise space based photometric follow-up of the system with ESAs
CHaracterising ExOplanets Satellite (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 (Teq​=1442−40+46​ K) implying a cloud-free atmosphere. TOI-5126 c is a warm
Neptune (Teq​=971−27+31​ 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.Comment: Accepted in MNRAS, 18 pages, 14 figure