The orbital alignment of HD 332231 b.

Contrary to the orthodox picture of planet formation resulting in a neatly ordered Solar System, exoplanet systems show a large diversity of orbits: short and long periods, circular and eccentric, well- and misaligned, even retrograde orbits. In order to understand this diversity it is essential to probe key orbital parameters. Spin-orbit alignment is such a parameter and can allude to the formation and migration history of the system. However, tidal circularization and alignment might hamper the interpretation of orbital eccentricity and obliquities in the context of planet formation and evolution for planets on orbits with less than about ten days. Here we aim to measure the projected stellar obliquity in the HD 332231 system in which a warm (period~18.7 days) giant planet orbits a bright F star on a circular orbit. We observed the system during a transit with the HARPS-N spectrograph and obtained data on the Rossiter-McLaughlin effect. We analyze the spectroscopic transit data together with new TESS photometry employing three different analysis methods. The results from the different approaches are fully consistent. We find a projected obliquity of -1+/-7 deg, indicating the stellar spin axis to be well-aligned with the orbit of the planet. We furthermore find evidence for Transit Timing Variations suggesting the presence of an additional third body in the system. Together with the low orbital eccentricity the good alignment suggests that this warm giant planet has not undergone high eccentricity migration.