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Extreme temperature contrasts between the day and nightside of ultra-hotJupiters result in significantly asymmetric atmospheres, with a large expansionoccurring over a small range of longitude around the terminator. Over thecourse of a transit, WASP-76b rotates by about 30 degree, changing theobservable part of the atmosphere and invoking variations in the appearance ofits constituents. As recently reported, this results in time-variable effectsin the neutral iron signal, which are amplified by its possible condensation onthe nightside. Here, we study the presence of molecular signals during atransit of WASP-76b observed with the CARMENES spectrograph and compare thecontributions from this planet's morning and evening terminators. The resultsare somewhat puzzling, with formal detections of water vapor (5.5$\sigma$) andhydrogen cyanide (5.2$\sigma$) but at significantly different positions in theK$_p$-V$_{sys}$ diagram, with a blueshift of -14.3 $\pm$ 2.6 km/s and aredshift of $+$20.8 $^{+7.8}_{-3.9}$ km/s respectively, and a higher K$_p$ thanexpected. The H$_2$O signal also appears stronger later on in the transit, incontrast to that of HCN, which seems stronger early on. We tentatively explainthis by silicate clouds forming and raining out on the nightside, partiallyremoving oxygen from the upper atmosphere. For C/O values between 0.7 and 1,this leads to the formation of HCN at the morning limb. At the eveningterminator, with the sequestered oxygen being returned to the gas phase due toevaporation, these C/O values lead to formation of H$_2$O instead of HCN. Ifconfirmed, these results indicate that individual molecules trace differentparts of the atmosphere, as well as nightside condensation, allowing spatialcharacterization. As these results are based on a single transit, we advocatethat more data are needed to confirm them and further explore these scenarios.

Searching for the origin of the Ehrenreich effect in ultra-hot Jupiters: Evidence for strong C/O gradients in the atmosphere of WASP-76b?