  In this work we quantify the effect of an unresolved companion star on the
derived stellar parameters of the primary star if a blended spectrum is fit
assuming the star is single. Fitting tools that determine stellar parameters
from spectra typically fit for a single star, but we know that up to half of
all exoplanet host stars may have one or more companion stars. We use
high-resolution spectra of planet host stars in the Kepler field from the
California-Kepler Survey to create simulated binaries; we select 8 stellar
pairs and vary the contribution of the secondary star, then determine stellar
parameters with SpecMatch-Emp and compare them to the parameters derived for
the primary star alone. We find that in most cases the effective temperature,
surface gravity, metallicity, and stellar radius derived from the composite
spectrum are within 2-3 $\sigma$ of the values determined from the unblended
spectrum, but the deviations depend on the properties of the two stars.
Relatively bright companion stars that are similar to the primary star have the
largest effect on the derived parameters; in these cases the stellar radii can
be overestimated by up to 60%. We find that metallicities are generally
underestimated, with values up to 8 times smaller than the typical uncertainty
in [Fe/H]. Our study shows that follow-up observations are necessary to detect
or set limits on stellar companions of planetary host stars so that stellar
(and planet) parameters are as accurate as possible.
