Context. Ultra-hot Jupiters have dayside temperatures at which most molecules are
expected to thermally dissociate. The dissociation of water vapour results in the production
of the hydroxyl radical (OH). While OH absorption is easily observed in near-infrared spectra
of M dwarfs, which have similar effective temperatures as ultra-hot Jupiters, it is often not
considered when studying the atmospheres of ultra-hot Jupiters. Ground-based high-
resolution spectroscopy during the primary transit is a powerful tool for detecting molecular …

Ultra-hot Jupiters have dayside temperatures at which most molecules are expected to
thermally dissociate. The dissociation of water vapour results in the production of the
hydroxyl radical (OH). We report on the detection of OH in the atmosphere of the ultra-hot
Jupiter WASP-76b using high-resolution transmission spectroscopy with CARMENES. We
find that the signal is blueshifted with-13 km/s, indicating that it mostly originates from the
hotter evening side of the planet, and is under the influence of a strong day-to-nightside …