  Surveys of nearby field stars indicate that stellar binaries are common, yet
little is known about the effects that these companions may have on planet
formation and evolution. The Friends of Hot Jupiters project uses three
complementary techniques to search for stellar companions to known
planet-hosting stars: radial velocity monitoring, adaptive optics imaging, and
near-infrared spectroscopy. In this paper, we examine high-resolution K band
infrared spectra of fifty stars hosting gas giant planets on short-period
orbits. We use spectral fitting to search for blended lines due to the presence
of cool stellar companions in the spectra of our target stars, where we are
sensitive to companions with temperatures between 3500-5000 K and projected
separations less than 100 AU in most systems. We identify eight systems with
candidate low-mass companions, including one companion that was independently
detected in our AO imaging survey. For systems with radial velocity
accelerations, a spectroscopic non-detection rules out scenarios involving a
stellar companion in a high inclination orbit. We use these data to place an
upper limit on the stellar binary fraction at small projected separations, and
show that the observed population of candidate companions is consistent with
that of field stars and also with the population of wide-separation companions
detected in our previous AO survey. We find no evidence that spectroscopic
stellar companions are preferentially located in systems with short-period gas
giant planets on eccentric and/or misaligned orbits.

Earth and Planetary Astrophysics