SOPHIE velocimetry of Kepler transit candidates^⋆

III. KOI-423b: an 18 M_Jup transiting companion around an F7IV star

¹ Institut d’Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France
e-mail: bouchy@iap.fr
² Observatoire de Haute-Provence, Université d’Aix-Marseille & CNRS, 04670 Saint-Michel l’Observatoire, France
³ Laboratoire d’Astrophysique de Marseille, Université Aix-Marseille, CNRS, 38 rue Frédéric Joliot-Curie, 13388 Marseille, France
⁴ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble, UMR 5274, 38041 Grenoble, France
⁵ Université de Nice-Sophia Antipolis, CNRS UMR 6202, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France
⁶ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland

Received: 17 April 2011
Accepted: 20 July 2011

Abstract

We report the strategy and results of our radial velocity follow-up campaign with the SOPHIE spectrograph (1.93-m OHP) of four transiting planetary candidates discovered by the Kepler space mission. We discuss the selection of the candidates KOI-428, KOI-410, KOI-552, and KOI-423. KOI-428 was established as a hot Jupiter transiting the largest and the most evolved star discovered so far and is described by Santerne et al. (2011, A&A, 528, A63). KOI-410 does not present radial velocity change greater than 120 m s^-1, which allows us to exclude at 3σ a transiting companion heavier than 3.4 M_Jup. KOI-552b appears to be a transiting low-mass star with a mass ratio of 0.15. KOI-423b is a new transiting companion in the overlapping region between massive planets and brown dwarfs. With a radius of 1.22 ± 0.11 R_Jup and a mass of 18.0 ± 0.92 M_Jup, KOI-423b is orbiting an F7IV star with a period of 21.0874 ± 0.0002 days and an eccentricity of 0.12 ± 0.02. From the four selected Kepler candidates, at least three of them have a Jupiter-size transiting companion, but two of them are not in the mass domain of Jupiter-like planets. KOI-423b and KOI-522b are members of a growing population of known massive companions orbiting close to an F-type star. This population currently appears to be absent around G-type stars, possibly due to their rapid braking and the engulfment of their companions by tidal decay.