We present precise radial velocities of XO-2 taken with the Subaru HDS, covering two transits of XO-2b with an interval of nearly two years. The data suggest that the orbital eccentricity of XO-2b is consistent with zero within 2$\sigma$ ($e=0.045\pm0.024$) and the orbit of XO-2b is prograde (the sky-projected spin-orbit alignment angle $\lambda=10^{ \circ}\pm72^{ \circ}$). The poor constraint of $\lambda$ is due to a small impact parameter (the orbital inclination of XO-2b is almost 90$^{ \circ}$). The data also provide an improved estimate of the mass of XO-2b as $0.62\pm0.02$ $M_{ \rm Jup}$. We also find a long-term radial velocity variation in this system. Further radial velocity measurements are necessary to specify the cause of this additional variation.
Categories: astro-ph.EP astro-ph.SR