SOPHIE velocimetry of Kepler transit candidates. VI. An additional companion in the KOI-13 system

We report the discovery of a new stellar companion in the KOI-13 system. KOI-13 is composed of two fast-rotating A-type stars of similar magnitude. One of these two stars hosts a transiting planet discovered by Kepler. We obtained new radial velocity measurements using the SOPHIE spectrograph at the Observatoire de Haute-Provence that reveal an additional companion in this system. This companion has a mass of between 0.4 M_⊙ and 1 M_⊙ and orbits one of the two main stars with a period of 65.831 ± 0.029 days and an eccentricity of 0.52 ± 0.02. The radial velocities of the two stars are derived using a model of two fast-rotating line profiles. From the residuals, we find a hint of the stellar variations seen in the Kepler light curve, which have an amplitude of about 1.41 km s^-1 and a period close to the rotational period. This signal appears to be about three orders of magnitude larger than expected for stellar activity. From the analysis of the residuals, we also put a 3-σ upper limit on the mass of the transiting planet KOI-13.01 of 14.8 M_Jup and 9.4 M_Jup, depending on which star hosts the transit. We find that this new companion has no significant impact on the photometric determination of the mass of KOI-13.01 but is expected to affect precise infrared photometry. Finally, using dynamical simulations, we infer that the new companion is orbiting around KOI-13B and that the transiting planet candidate is expected to orbit KOI-13A. Thus, the transiting planet candidate KOI-13.01 is orbiting the main component of a hierarchical triple system.

Based on observations made with SOPHIE on the 1.93-m telescope at Observatoire de Haute-Provence (CNRS), France.Table 1 (RV data) is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/544/L12