  There has been increasing progress toward detailed characterization of
exoplanetary atmospheres, in both observations and theoretical methods.
Improvements in observational facilities and data reduction and analysis
techniques are enabling increasingly higher quality spectra, especially from
ground-based facilities. The high data quality also necessitates concomitant
improvements in models required to interpret such data. In particular, the
detection of trace species such as metal oxides has been challenging. Extremely
irradiated exoplanets (~3000 K) are expected to show oxides with strong
absorption signals in the optical. However, there are only a few hot Jupiters
where such signatures have been reported. Here we aim to characterize the
atmosphere of the ultra-hot Jupiter WASP-33b using two primary transits taken
18 orbits apart. Our atmospheric retrieval, performed on the combined data
sets, provides initial constraints on the atmospheric composition of WASP-33b.
We report a possible indication of aluminum oxide (AlO) at 3.3-sigma
significance. The data were obtained with the long slit OSIRIS spectrograph
mounted at the 10-meter Gran Telescopio Canarias. We cleaned the brightness
variations from the light curves produced by stellar pulsations, and we
determined the wavelength-dependent variability of the planetary radius caused
by the atmospheric absorption of stellar light. A simultaneous fit to the two
transit light curves allowed us to refine the transit parameters, and the
common wavelength coverage between the two transits served to contrast our
results. Future observations with HST as well as other large ground-based
facilities will be able to further constrain the atmospheric chemical
composition of the planet.
