Abstract

HIP41378 f is a temperate 9.2 +/- 0.1 R-circle plus planet with period of 542.08 d and an extremely low density of 0.09 +/- 0.02 g cm(-3). It transits the bright star HIP41378 (V = 8.93), making it an exciting target for atmospheric characterization including transmission spectroscopy. HIP41378 was monitored photometrically between the dates of 2019 November 19 and 28. We detected a transit of HIP41378 f with NGTS, just the third transit ever detected for this planet, which confirms the orbital period. This is also the first ground-based detection of a transit of HIP41378 f. Additional ground-based photometry was also obtained and used to constrain the time of the transit. The transit was measured to occur 1.50 h earlier than predicted. We use an analytic transit timing variation (TTV) model to show the observed TTV can be explained by interactions between HIP41378e and HIP41378f. Using our TTV model, we predict the epochs of future transits of HIP41378f, with derived transit centres of T-C,T-4 = 2459 355.087(-0.022)(+0.031) (2021 May) and T-C,T-5 = 2459 897.078(-0.060)(+0.114) (2022 November).