tokens: dissociation of TiO and VO in the limb even at the lower altitudes probed by our data, reducing the log VMR of both species to −10, well below the detection limits we set with our injection tests. It is also interesting to note that the work of Evans et al. (2017 , 2018 ) and Mikal-Evans et al. (2019) infers the presence of VO, but not TiO, when both were predicted as inversion-drivers by Hubeny et al. (2003) and Fortney et al. (2008) and were largely expected to occur together. It has been shown in the models of Goyal et al. (2019) that greater abundance of VO over TiO is only expected in a very narrow temperature range: ~ 1700–1800 K assuming rainout condensation, or ~1200–1400 K assuming local condensation. This highlights the importance of robust detections and non-detections of TiO and VO as a diagnostic for temperature and condensation processes in the atmospheres of ultra-hot Jupiters. Despite our non-detection of TiO and VO, Evans et al. (2018) still show a feature in their transmission spectra at 600–800 nm consistent with the presence of VO (or perhaps another molecule absorbing strongly in this range). Additionally, their emission spectra ( Evans et al. 2017 ; Mikal-Evans et al. 2019 ) are still best-fit with models containing VO and clearly show water emission features consistent with the presence of a temperature inversion. It was theorised by Spiegel et al. (2009) that a hot Jupiter witha solar carbon-to-oxygen ratio would require TiO of at least solar abundance to cause a temperature inversion, a level which both we and Evans et al. (2018) rule out in our analysis. Our analysis in this paper now seems to rule out the presence of VO also, although again we emphasise that VO cannot be robustly ruled out using high-resolution cross-correlation methodology until a more suitably accurate line list is published. Nevertheless, although TiO and VO were long thought to be the predominant cause of temperature inversions in hot Jupiters, it may now be time to look for other culprits. Recent theoretical work has postulated a range of potential optical absorbers such as SiO, Fe, Mg, AlO, CaO and metal hydrides ( Lothringer et al. 2018 ; Lothringer Barman 2019 ; Gandhi Madhusudhan 2019 ), some of which (such as AlO) have absorption bands in the 600–800 nm range. Gandhi Madhusudhan (2019) also suggested a theoretical link between the C/O ratio and the presence of a temperature inversion, postulating that a reduced infrared opacity due to a low H 2 O abundance in WASP-121b could contribute towards the existence of an inversion layer; the presence of H 2 O emission features, however, would appear to rule out this possibility. Finally, the transmission spectrum presented by Evans et al. (2018) shows a strong upward trend in the blue optical much steeper than would be consistent with Rayleigh scattering, which they tentatively identify as due to the presence of SH, another molecule known to absorb strongly in the optical and thus
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