  We report the discovery of a new stellar companion in the KOI-13 system.
KOI-13 is composed by 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 revealed an additional companion in this
system. This companion has a mass between 0.4 and 1 Msun and orbits one of the
two main stars with a period of 65.831 \pm 0.029 days and an eccentricity of
0.52 \pm 0.02. The radial velocities of the two stars were derived using a
model of two fast-rotating line profiles. From the residuals, we found a hint
of the stellar variations seen in the Kepler light curve with an amplitude of
about 1.41 km/s and a period close to the rotational period. This signal
appears to be about three order of magnitude larger than expected for stellar
activity. From the analysis of the residuals, we also put a 3-sigma upper-limit
on the mass of the transiting planet KOI-13.01 of 14.8 Mjup and 9.4 Mjup,
depending on which star hosts the transit. We found 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 while
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.
