Variable Stars in Sagittarius Globular Clusters I. Arp~2
  We present new photometry and analysis of the twelve variable stars (nine RR
Lyrae, three SX Phoenicis) belonging to the Sagittarius globular cluster Arp 2.
Of the nine RR Lyrae stars in the cluster, eight are RRab and one is RRc. From
the RRab stars, we determined a mean period of $\langle
P_{ab}\rangle=0.581\pm0.047$ days, where the error is the standard error of the
mean. This places Arp 2 at the border between the Oosterhoff I and
Oosterhoff-Int clusters. Using the $V$-band data from the RR Lyrae stars, a
distance modulus of $(m-M)_0=17.24\pm0.17$ was determined. From the $I$-band
data, we found $(m-M)_0=17.34\pm0.07$. We also used the SX Phoenicis variables
to determine a distance modulus of $(m-M)_0=17.27\pm0.04$. Color excesses were
determined from the RR Lyrae light curves using both the ($B-V$) and ($V-I$)
colors. The mean reddening values were in line with or were a little higher
than those found in the literature. Both methods indicated star-to-star
variability in the reddening toward Arp 2. Of the nine RR Lyrae stars, seven
were flagged as variables by Gaia, with three having periods determined. We
used the Gaia data to investigate the membership of the seven Gaia RR Lyrae.
Although Arp 2 is too distant for reliable Gaia parallax, the current data do
not exclude any of the variables discussed in this paper from being members of
Arp 2.
