XO-2b: a Prograde Planet with Negligible Eccentricity and an Additional Radial Velocity Variation

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 +/- 0.024), and that the orbit of XO-2b is prograde (the sky-projected spin orbit alignment angle lambda = 10 degrees +/- 72 degrees). The poor constraint of lambda is due to a small impact parameter (the orbital inclination of XO-2b is almost 90 degrees). The data also provide an improved estimate of the mass of XO-2b as 0.62 +/- 0.02 M-Jup. Further, we find a long-term radial velocity variation in this system. Additional radial velocity measurements are necessary to specify the cause of this additional variation.