We discuss the discovery and characterization of the circumbinary planet Kepler-38b .
The stellar binary is single-lined , with a period of 18.8 days , and consists of a moderately evolved main-sequence star ( M _ { A } = 0.949 \pm 0.059 M _ { \odot } and R _ { A } = 1.757 \pm 0.034 R _ { \odot } ) paired with a low-mass star ( M _ { B } = 0.249 \pm 0.010 M _ { \odot } and R _ { B } = 0.2724 \pm 0.0053 R _ { \odot } ) in a mildly eccentric ( e = 0.103 ) orbit .
A total of eight transits due to a circumbinary planet crossing the primary star were identified in the Kepler light curve ( using Kepler Quarters 1 through 11 ) , from which a planetary period of 105.595 \pm 0.053 days can be established .
A photometric dynamical model fit to the radial velocity curve and Kepler light curve yields a planetary radius of 4.35 \pm 0.11 R _ { \oplus } , or 1.12 \pm 0.03 R _ { Nep } .
Since the planet is not sufficiently massive to observably alter the orbit of the binary from Keplerian motion , we can only place an upper limit on the mass of the planet of 122 M _ { \oplus } ( 7.11 M _ { Nep } or 0.384 M _ { Jup } ) at 95 % confidence .
This upper limit should decrease as more Kepler data become available .