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SOPHIE velocimetry of Kepler transit candidates
XII. KOI-1257 b: a highly eccentric three-month period transiting exoplanet

A. Santerne, G. HÚbrard, M. Deleuil, M. Havel, A. C. M. Correia, J.-M. Almenara, R. Alonso, L. Arnold, S. C. C. Barros, R. Behrend, L. Bernasconi, I. Boisse, A. S. Bonomo, F. Bouchy, G. Bruno, C. Damiani, R. F. DÝaz, D. Gravallon, T. Guillot, O. Labrevoir, G. Montagnier, C. Moutou, C. Rinner, N. C. Santos, L. Abe, M. Audejean, P. Bendjoya, C. Gillier, J. Gregorio, P. Martinez, J. Michelet, R. Montaigut, R. Poncy, J.-P. Rivet, G. Rousseau, R. Roy, O. Suarez, M. Vanhuysse, D. Verilhac

Abstract
In this paper we report a new transiting warm giant planet: KOI-1257 b. It was first detected in photometry as a planet-candidate by the Kepler space telescope and then validated thanks to a radial velocity follow-up with the SOPHIE spectrograph. It orbits its host star with a period of 86.647661 d ± 3 s and a high eccentricity of 0.772 ± 0.045. The planet transits the main star of a metal-rich, relatively old binary system with stars of mass of 0.99 ± 0.05 M and 0.70 ± 0.07 M for the primary and secondary, respectively. This binary system is constrained thanks to a self-consistent modelling of the Kepler transit light curve, the SOPHIE radial velocities, line bisector and full-width half maximum (FWHM) variations, and the spectral energy distribution. However, future observations are needed to confirm it. The PASTIS fully-Bayesian software was used to validate the nature of the planet and to determine which star of the binary system is the transit host. By accounting for the dilution from the binary both in photometry and in radial velocity, we find that the planet has a mass of 1.45 ± 0.35 M , and a radius of 0.94 ± 0.12 R , and thus a bulk density of 2.1 ± 1.2 g cm-3. The planet has an equilibrium temperature of 511 ± 50 K, making it one of the few known members of the warm-Jupiter population. The HARPS-N spectrograph was also used to observe a transit of KOI-1257 b, simultaneously with a joint amateur and professional photometric follow-up, with the aim of constraining the orbital obliquity of the planet. However, the Rossiter-McLaughlin effect was not clearly detected, resulting in poor constraints on the orbital obliquity of the planet.

Keywords
planetary systems – techniques: photometric – techniques: radial velocities – techniques: spectroscopic – methods: data analysis – stars: individual: KOI-1257 Ab

Astronomy and Astrophysics
Volume 571, Number of pages A37_1
2014 November

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Instituto de Astrof├şsica e Ci├¬ncias do Espa├žo Universidade do Porto Faculdade de Ciências da Universidade de Lisboa Funda├ž├úo para a Ci├¬ncia e a Tecnologia
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