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We observed the hot Jupiter HAT-P-32b (also known as HAT-P-32Ab) to determineits optical transmission spectrum by measuring the wavelength-dependentplanet-to-star radius ratios in the region between 518 - 918 nm. We used theOSIRIS instrument at the GTC in long slit spectroscopy mode, placing HAT-P-32and a reference star in the same slit and obtaining a time series of spectracovering two transit events. Using the best quality data set, we were able toyield 20 narrow-band transit light curves, with each passband spanning a 20 nmwide interval. After removal of all systematic noise signals and light curvemodeling the uncertainties for the resulting radius ratios lie between 337 and972 ppm. The radius ratios show little variation with wavelength suggesting ahigh altitude cloud layer masking any atmospheric features. Alternatively, astrong depletion in alkali metals or a much smaller than expected planetaryatmospheric scale height could be responsible for the lack of atmosphericfeatures. Our result of a flat transmission spectrum is consistent with aprevious ground-based study of the optical spectrum of this planet. Thisagreement between independent results demonstrates that ground-basedmeasurements of exoplanet atmospheres can give reliable and reproducibleresults despite the fact that the data often is heavily affected by systematicnoise, as long as the noise source is well understood and properly corrected.We also extract an optical spectrum of the M-dwarf companion HAT-P-32B. UsingPHOENIX stellar atmosphere models we determine an effective temperature of$T_\mathrm{eff}= 3187^{+60}_{-71}$ K, slightly colder than previous studiesrelying only on broadband infra-red data.

The GTC exoplanet transit spectroscopy survey. IV. Confirmation of the flat transmission spectrum of HAT-P-32b