WASP-33 is a hard system to analyse since the host star is a delta-Scuti star, which means that it pulsates. That produces transit lightcurves like these, where the usual transit profile has pulsations superimposed on it. The figure shows the transit in different wavebands across the optical, from blue to red, as obtained with the OSIRIS spectrograph. That meant that the authors first had to model and subtract the effect of the pulsations.
After doing that they analysed how the transit depth depended on wavelength, which reveals how the planet’s atmosphere absorbs light. “We find that the feature observed between 450 and 550 nm can best be explained by aluminium oxide in its atmosphere” says lead author, Carolina von Essen.
“The current models of exoplanetary atmospheres predict that the Ultra Hot Jupiters should be free of clouds, and present a range of oxides in the visible spectrum, such as vanadium oxide, titanium oxide, and aluminium oxide”. This work on WASP-33b is the first observational indication of the presence of aluminium oxide.