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Feature-rich transmission spectrum for WASP-127b: Cloud-free skies for the puffiest known super-Neptune?

Context. WASP-127b is a planet with one of the lowest densities discovered to date. With a sub-Saturn mass ( M p = 0.18 ± 0.02 M J ) and super-Jupiter radius ( R p = 1.37 ± 0.04 R J ), it orbits a bright G5 star that is about to leave the main-sequence. Aims. We aim to explore the atmosphere of WASP-127b in order to retrieve its main atmospheric components, and to find hints for its intriguing inflation and evolutionary history. Methods. We used the ALFOSC spectrograph at the NOT telescope to observe a low-resolution ( R ~ 330, seeing limited) long-slit spectroscopic time series during a planetary transit, and present here the first transmission spectrum for WASP-127b. Results. We find a strong Rayleigh slope at blue wavelengths and a hint of Na absorption, although the quality of the data does not allow us to claim a detection. At redder wavelengths the absorption features of TiO and VO are the best explanation to fit the data. Conclusions. Although observations with a higher signal-to-noise ratio are needed to conclusively confirm the absorption features, WASP-127b seems to posses a cloud-free atmosphere and is one of the best targets on which to perform further characterization studies in the near future.