We report the discovery of the planet XO-4b , which transits the star XO-4Â ( GSC 03793-01994 , V=10.7 , F5V ) .
Transits are 1.0 % deep and 4.4 hours in duration .
The star XO-4 has a mass of 1.32 \pm 0.02 M _ { \odot } , a radius of 1.55 \pm 0.05 R _ { \odot } , an age of 2.1 \pm 0.6 Gyr , a distance of 293 \pm 19 pc , an effective temperature of 5700 \pm 70 K , a logarithmic iron abundance of -0.04 \pm 0.03 relative to solar , a logarithmic surface gravity in cgs units of 4.18 \pm 0.07 , and a projected rotational velocity of 8.8 \pm 0.5 km s ^ { -1 } .
The star XO-4 has periodic radial velocity variations with a semi-amplitude of 163 \pm 16 m s ^ { -1 } , due to the planet XO-4b .
The planet XO-4b has a mass of 1.72 \pm 0.20 M _ { J } , a radius of 1.34 \pm 0.048 R _ { J } , an orbital period of 4.12502 \pm 0.00002 days , and a heliocentric Julian date at mid-transit of 2454485.9322 \pm 0.0004 .
We analyze scintillation-limited differential R-band photometry of XO-4b in transit made with a 1.8-m telescope under photometric conditions , yielding photometric precision of 0.6 to 2.0 millimag per one-minute interval .
The declination of XO-4Â places it within the continuous viewing zone of the Hubble Space Telescope ( HST ) , which permits observation without interruption caused by occultation by the Earth .
Because the stellar rotation periods of the three hottest stars orbited by transiting gas-giant planets are 2.0 , 1.1 , and 2.0 times the planetary orbital periods , we note the possibility of resonant interaction .