We report a measurement of the Rossiter–McLaughlin effect in the transiting extrasolar planetary system TrES-1 , via simultaneous spectroscopic and photometric observations with the Subaru and MAGNUM telescopes .
By modeling the radial velocity anomaly that was observed during a transit , we determine the sky-projected angle between the stellar spin axis and the planetary orbital axis to be \lambda = 30 \pm 21 [ deg ] .
This is the third case for which \lambda has been measured in a transiting exoplanetary system , and the first demonstration that such measurements are possible for relatively faint host stars ( V \sim 12 , as compared to V \sim 8 for the other systems ) .
We also derive a time of mid-transit , constraints on the eccentricity of the TrES-1b orbit ( e = 0.048 \pm 0.025 ) , and upper limits on the mass of the Trojan companions ( \lesssim 14 M _ { \oplus } ) at the 3 \sigma level .