We report on a V=11.2 early K dwarf , XO-2 ( GSC 03413-00005 ) , that hosts a R _ { p } =0.98 \pm ^ { 0.03 } _ { 0.01 } { R _ { J } } , M _ { p } =0.57 \pm 0.06 { M _ { J } } transiting extrasolar planet , XO-2b , with an orbital period of 2.615857 \pm 0.000005 days .
XO-2 has high metallicity , [ Fe/H ] =0.45 \pm 0.02 , high proper motion , \mu _ { tot } = 157 mas yr ^ { -1 } , and has a common proper motion stellar companion with 31 \arcsec separation .
The two stars are nearly identical twins , with very similar spectra and apparent magnitudes .
Due to the high metallicity , these early K dwarf stars have a mass and radius close to solar , M _ { \star } = 0.98 \pm 0.02 { M _ { \odot } } and R _ { \star } = 0.97 \pm ^ { 0.02 } _ { 0.01 } { R _ { \odot } } .
The high proper motion of XO-2 results from an eccentric orbit ( Galactic pericenter , R _ { per } < 4 kpc ) well confined to the Galactic disk ( Z _ { max } \sim 100 pc ) .
In addition , the phase space position of XO-2 is near the Hercules dynamical stream , which points to an origin of XO-2 in the metal-rich , inner Thin Disk and subsequent dynamical scattering into the solar neighborhood .
We describe an efficient Markov Chain Monte Carlo algorithm for calculating the Bayesian posterior probability of the system parameters from a transit light curve .