in the atmosphere, and the energy is redistributed better; they harbor no temperature inversion and have a lower day-night temperature contrast. The mixing ratio of TiO and VO in the atmosphere becomes smaller as the temperature decreases, leading atmospheric properties to change gradually between both classes. Given its substellar flux, XO-2b lies in this predicted transition region. Indeed, atmospheric potassium is a characteristic of pL class planets, whereas temperature inversion is expected for pM class planets. Models also predict that planets with a relatively low substellar flux would possess water features in absorption both on the dayside and at the terminator [@Fortney2008; @Fortney2010].

On a dynamical point of view, XO-2b belongs to the class of close-in giant planets spin-orbit aligned and orbiting prograde [@Narita2011]. Their radial velocity measurements are consistent with a null eccentricity. They identify a long term radial velocity variation in one of the epochs, which could be due to a hypothetical third body in the system.

The HST NICMOS observations of XO-2b presented here may be compared to those of four exoplanets, HD 189733b, HD 209458b, XO-1b, and GJ 436b. Water and methane were detected in the atmosphere of HD 189733b [@Swain2008]. Carbon monoxide was expected but 