The changing transit shape of TOI-3884 b

¹ Geneva Observatory, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ Institute of Physics, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, Chemin Pegasi 51b, 1290 Versoix, Switzerland
⁴ Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland

^★ Corresponding author: Hritam.Chakraborty@unige.ch

Received: 17 June 2025
Accepted: 4 September 2025

Abstract

TOI-3884 b is a sub-Saturn transiting a fully convective M dwarf. Observations indicate that the transit shape is chromatic and asymmetric as a result of persistent starspot crossings. This, along with the lack of high-amplitude photometric variability of the host star, indicates that the rotational axis of the star is tilted along our line of sight and the planet-occulted starspot is located close to the stellar pole. We acquired photometric transits over a period of three years with the Swiss 1.2-metre Euler telescope to track changes in the starspot configuration and detect any signs of decay or growth. The shape of the transit changes over time, and so far no two observations match perfectly. We conclude that the observed variability is likely not caused by changes in the temperature and size of the spot, but due to a slight misalignment (5.64 ± 0.64°) between the spot centre and the stellar pole, i.e. a small spin-spot angle (Θ). In addition, we were able to obtain precise measurements of the sky-projected spin-orbit angle (λ) of 37.3 ± 1.5°, and the true spin-orbit angle (ψ) of 54.3 ± 1.4°. The precise alignment measurements along with future atmospheric characterisation with the James Webb Space Telescope will be vital for understanding the formation and evolution of close-in, massive planets around fully convective stars.