Variability in NGC 3201 giant stars and its impact on their
  spectroscopic [Fe/H] determination
  We present the analysis of 34 light curves in $V$ and $I$ of 17 giant stars
in the globular cluster NGC 3201, to check if such stars are variable and if
their variability has some kind of impact on the iron abundance as obtained
from spectroscopic measurements. First, we computed the Generalized
Lomb-Scargle and Phase Dispersion Minimization periodograms on the sample to
check if the stars were variables. In this way, 7 stars of the sample were
found to be non-variable, 2 stars are considered as possible variables, and 8
stars were found to be variable, with periods ranging from $0.0881\pm0.0001$ to
$0.5418\pm0.0027$ days. According to the literature, the variables have
distinct values of $\text{[Fe I/H]}$: the 3 most metal-rich stars are in the
RGB stage, one has an $\text{[Fe I/H]}=-1.37$ dex, while the other two have
$\text{[Fe I/H]}=-1.31$ dex. The two most metal-poor variables have $\text{[Fe
I/H]}=-1.61$ dex and $\text{[Fe I/H]}=-1.62$ dex, and are AGB stars; the
remaining variables have $\text{[Fe I/H]}=-1.44$, $-1.48$, and $-1.50$ dex, the
first two being RGB while the last is AGB star. On the other hand, stars that
appear to be non-variables have $-1.56\leq\text{[Fe I/H]}\leq-1.40$. We
conclude that variability somehow affects the spectroscopic determination of
the iron content of giant stars in NGC 3201 increasing the iron spread of the
cluster. If variability is not taken into account, this spread could be wrongly
interpreted as due to an intrinsic iron spread affecting the stars of the
cluster.
