XO-2b: a Prograde Planet with a 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$\sigma$ ($e=0.045\pm0.024$) 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. 
Norio Narita, Teruyuki Hirano, Bun'ei Sato, Hiroki Harakawa, Akihiko   Fukui, Wako Aoki, Motohide Tamura
astro-ph.EP astro-ph.SR