hot Jupiter with a radius $0.996 \;\rm R_{jup}$ and mass $0.565 \rm \;M_{jup}$ [@Burke2007]. The host star is an early K0 V dwarf of magnitude H = 9.34, radius and mass close to solar, and metallicity Fe/H = 0.45. A companion star is located at 31" separation ([$\sim$]{}4600 AU at a distance d [$\sim$]{}150 pc), and has an apparent magnitude H = 9.37, a metallicity Fe/H = 0.47, and a common proper motion $\mu$ = 157 mas yr$^{-1}$. Operationally, the presence of such a close-by twin companion provides an ideal comparison star.

Subsequent transit observations were conducted by [@Fernandez2009], gathering 6 transits of XO-2b. By combining the lightcurve analysis with theoretical isochrones of stellar evolution models, parameters consistent with those of [@Burke2007] were found for the planet and host star. Only the period differs by 2.5 $\sigma$. This discrepancy can be explained by an additional body orbiting XO-2, or simply from an underestimation of uncertainties. Secondary eclipse measurements were taken on XO-2b with IRAC on the Spitzer Space Telescope [@Machalek2009]. They provide a tentative evidence for a weak temperature inversion in the upper atmosphere of the planet and a dayside temperature $T \sim 1500$ K, and predicted a substellar flux 