  With the aim of connecting the compositions of stars and planets, we present
the abundances of carbon and oxygen, as well as iron and nickel, for the
transiting exoplanet host star XO-2N and its wide-separation binary companion
XO-2S. Stellar parameters are derived from high-resolution,
high-signal-to-noise spectra, and the two stars are found to be similar in
their Teff, log g, iron ([Fe/H]), nickel ([Ni/H]) abundances. Their carbon
([C/H]) and oxygen ([O/H]) abundances also overlap within errors, although
XO-2N may be slightly more C-rich and O-rich than XO-2S. The C/O ratios of both
stars (~0.60+/-0.20) may also be somewhat larger than solar (C/O~0.50). The
XO-2 system has a transiting hot Jupiter orbiting one binary component but not
the other, allowing us to probe the potential effects planet formation might
have on the host star composition. Additionally, with multiple observations of
its atmosphere the transiting exoplanet XO-2b lends itself to compositional
analysis, which can be compared to the natal chemical environment established
by our binary star elemental abundances. This work sets the stage for
determining how similar/different exoplanet and host star compositions are, and
the implications for planet formation, by discussing the C/O ratio measurements
in the unique environment of a visual binary system with one star hosting a
transiting hot Jupiter.
