  The transiting hot Jupiter XO-2b is an ideal target for multi-object
photometry and spectroscopy as it has a relatively bright ($V$-mag = 11.25) K0V
host star (XO-2N) and a large planet-to-star contrast ratio
(R$_{p}$/R$_{s}\approx0.015$). It also has a nearby (31.21") binary stellar
companion (XO-2S) of nearly the same brightness ($V$-mag = 11.20) and spectral
type (G9V), allowing for the characterization and removal of shared systematic
errors (e.g., airmass brightness variations). We have therefore conducted a
multiyear (2012--2015) study of XO-2b with the University of Arizona's 61"
(1.55~m) Kuiper Telescope and Mont4k CCD in the Bessel U and Harris B
photometric passbands to measure its Rayleigh scattering slope to place upper
limits on the pressure-dependent radius at, e.g., 10~bar. Such measurements are
needed to constrain its derived molecular abundances from primary transit
observations. We have also been monitoring XO-2N since the 2013--2014 winter
season with Tennessee State University's Celestron-14 (0.36~m) automated
imaging telescope to investigate stellar variability, which could affect
XO-2b's transit depth. Our observations indicate that XO-2N is variable,
potentially due to {cool star} spots, {with a peak-to-peak amplitude of $0.0049
\pm 0.0007$~R-mag and a period of $29.89 \pm 0.16$~days for the 2013--2014
observing season and a peak-to-peak amplitude of $0.0035 \pm 0.0007$~R-mag and
$27.34 \pm 0.21$~day period for the 2014--2015 observing season. Because of}
the likely influence of XO-2N's variability on the derivation of XO-2b's
transit depth, we cannot bin multiple nights of data to decrease our
uncertainties, preventing us from constraining its gas abundances. This study
demonstrates that long-term monitoring programs of exoplanet host stars are
crucial for understanding host star variability.
