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Rayleigh scattering in a hydrogen-dominated exoplanet atmosphere can bedetected from ground or space based telescopes, however, stellar activity inthe form of spots can mimic Rayleigh scattering in the observed transmissionspectrum. Quantifying this phenomena is key to our correct interpretation ofexoplanet atmospheric properties. We obtained long-slit optical spectroscopy oftwo transits of HAT-P-11b with the Optical System for Imaging andlow-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) at Gran TelescopioCanarias (GTC) on August 30 2016 and September 25 2017. We integrated thespectrum of HAT-P-11 and one reference star in several spectroscopic channelsacross the $\lambda\sim$ 400-785 nm region, creating numerous light curves ofthe transits. We fit analytic transit curves to the data taking into accountthe systematic effects and red noise present in the time series in an effort tomeasure the change of the planet-to-star radius ratio($R_\mathrm{p}/R_\mathrm{s}$) across wavelength. By fitting both transitstogether, we find a slope in the transmission spectrum showing an increase ofthe planetary radius towards blue wavelengths. A closer inspection to thetransmission spectrum of the individual data sets reveals that the firsttransit presents this slope while the transmission spectrum of the second dataset is flat. Additionally we detect hints of Na absorption in the first night,but not in the second. We conclude that the transmission spectrum slope and Naabsorption excess found in the first transit observation are caused byunocculted stellar spots. Modeling the contribution of unocculted spots toreproduce the results of the first night we find a spot filling factor of$\delta=0.62^{+0.20}_{-0.17}$ and a spot-to-photosphere temperature differenceof $\Delta T = 429^{+184}_{-299}$ K.

The GTC exoplanet transit spectroscopy survey X. Stellar spots versus Rayleigh scattering: the case of HAT-P-11b