  We study the variable star content of the globular cluster M53 to compute the
physical parameters of the constituting stars and the distance of the cluster.
Covering two adjacent seasons in 2007 and 2008, new photometric data are
gathered for 3048 objects in the field of M53. By using the OIS method and
subsequently TFA, we search for variables in the full sample by using DFT and
BLS methods. We select variables based on the statistics related to these
methods combined with visual inspections. We identified 12 new variables (2 RR
Lyrae stars, 7 short periodic stars - 3 of them are SX Phe stars - and 3
long-period variables). No eclipsing binaries were found in the present sample.
Except for the 3 (hitherto unknown) Blazhko RR Lyrae stars, no multiperiodic
variables were found. We showed that after proper period shift, the PLC
relation for the first overtone RR Lyrae sample tightly follows the one spanned
by the fundamental stars. Furthermore, the slope is in agreement with the one
derived from other clusters. Based on the earlier Baade-Wesselink calibration
of the PLC relations, the derived reddening-free distance modulus of M53 is
16.31 +/- 0.04 mag, corresponding to a distance modulus of 18.5 mag for the
Large Magellanic Cloud. From the Fourier parameters of the RRab stars we
obtained an average iron abundance of -1.58 +/- 0.03. This is ~0.5 dex higher
than the overall abundance of the giants as given in the literature and derived
in this paper from the three-color photometry of giants. We suspect that the
source of this discrepancy (observable also in other, low-metallicity clusters)
is the want of sufficient number of low-metallicity objects in the calibrating
sample of the Fourier method.
