of $0.76\times10^9$ erg cm$^{-2}$ s$^{-1}$. Optical ground-based narrowband transit photometry with the 10.4 m Gran Telescopio Canarias yields clues on the atmospheric composition of XO-2b. Three transits were observed, each time in 4 wavelengths (679.2, 758.2, 766.5, and 883.9 nm), enabling detection of potassium in the planetary atmosphere [@Sing2011]. This atmospheric component, as well as sodium, was predicted to be a dominant source of opacity at optical wavelengths for close-in giant planets such as XO-2b [@Seager2000; @Brown2001; @Hubbard2001]. Models predict that the opacity from K, Na, and H$_2$ Rayleigh scattering dominate the optical, while H$_2$O absorption should dominate beyond 900 nm. Finally, the planet and host star parameters derived from GTC data agree well with @Fernandez2009 [@Burke2007], and best fits to atmospheric models are obtained using a planet temperature of 1500 K.

It has been proposed that hot Jupiters can be separated into two classes based on the incident flux received from their host star [@Fortney2008; @Burrows2008]. The most irradiated planets, the pM class, are hot, harbor a temperature inversion due to the presence of TiO/VO gases in the upper atmosphere, and have a large day-night temperature contrast. The less irradiated planets, the pL class, are cooler, absorb flux deeper 