WASP-38b: a transiting exoplanet in an eccentric, 6.87d period orbit

Aims. We report the discovery of WASP-38b, a long period transiting planet in an eccentric 6.871815 day orbit. The transit epoch is 2   455   335.92050 ± 0.00074 (HJD) and the transit duration is 4.663 h. Methods. WASP-38b’s discovery was enabled due to an upgrade to the SuperWASP-North cameras. We performed a spectral analysis of the host star HD 146389/BD+10 2980 that yielded Teff = 6150 ± 80 K, log g = 4.3 ± 0.1, vsini = 8.6 ± 0.4 km s-1, M∗ = 1.16 ± 0.04 M⊙ and R∗ = 1.33 ± 0.03 R⊙, consistent with a dwarf of spectral type F8. Assuming a main-sequence mass-radius relation for the star, we fitted simultaneously the radial velocity variations and the transit light curves to estimate the orbital and planetary parameters. Results. The planet has a mass of 2.69 ± 0.06 MJup and a radius of 1.09 ± 0.03 RJup giving a density, ρp = 2.1 ± 0.1 ρJ. The high precision of the eccentricity e = 0.0314 ± 0.0044 is due to the relative transit timing from the light curves and the RV shape. The planet equilibrium temperature is estimated at 1292 ± 33 K. WASP-38b is the longest period planet found by SuperWASP-North and with a bright host star (V = 9.4 mag), is a good candidate for followup atmospheric studies.