tokens: is consistent with a solar value of log (Al∕H) = −5.55 ± 0.03 ( Asplund et al. 2009 ). However, Al can also be present in other refractory species, in which case our derived estimate is a lower limit on the true Al/H abundance in the atmosphere of WASP-33 b. In particular, the dominant Al-containing species in a solar-composition atmosphere in equilibrium, at temperatures near 3000 K, include Al and AlH ( Woitke et al. 2018 ). Under such conditions, our retrieved AlO abundance is ~ 10 3 × higher than that predicted in chemical equilibrium with solar abundances. Future studies may investigate the feasibility of our retrieved abundance of AlO in WASP-33 b, possibly due to chemical disequilibrium or other mechanisms. Future observations may also provide better constraints on the same. 4.2 The aluminum oxide feature The main spectral feature of the optical transmission spectrum reported in this work is an increase in the planet-star radius ratio blueward of about 560 nm. In general, an increase toward short wavelengths can be caused by third light of an additional stellar companion in the aperture of flux extraction. Moya et al. (2011) detected a stellar object 2 arcsec away from WASP-33, well included within our aperture. The authors estimated that the probable physical companion of WASP-33 should have an effective temperature of about ~3000 K. As carried out in von Essen et al. (2015) , within our wavelength region we estimated that the M dwarf should be at least 10 3 times dimmer than WASP-33. This would create a variability of planet-to-star radius ratio as large as 60 ppm. This upper value is well contained within our error bars given that the median precision of the dataset is 245 ppm. Thus, it is safe to neglect the contribution of WASP-33’s companion. Also, star spots might in principle cause a spectral slope in the transmission spectrum ( Sing et al. 2011 ; Oshagh et al. 2014 ). This possibility plays no role for WASP-33 either, since it has no or a too thin outer convection zone, making star spots unlikely to occur. In consequence, we have interpreted the spectral feature shortward of 560 nm as an increase in opacity in the planetary atmosphere of WASP-33 b. In Sect. 4.1 we retrieved AlO as the opacity source of the spectral feature between 450 and 550nm. Spectral absorption features by gaseous AlO have not been reported in the literature of exoplanetobservations, though atmospheric models predict its importance in hot Jupiters ( Gandhi Madhusudhan 2018b ). AlO was described as a potential condensate forming clouds or hazes in very hot exoplanet atmospheres (e.g., Lodders 2002 ; Wakeford et al. 2017 ; Pinhas Madhusudhan 2017 ). AlO is difficult to observe for M or L type stellar objects, first because the optical spectra of these low-mass stars are dominated by TiO and VO absorption features, and second because such spectral feature at ~ 500 nm is challenging to observe for very late type stars because of their intrinsic faintness at
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