Variable stars in the bulge globular cluster NGC 6401
  We present a study of variable stars in globular cluster NGC 6401. The
cluster is only $5.3\degr$ away from the Galactic centre and suffers from
strong differential reddening. The photometric precision afforded us by
difference image analysis resulted in improved sensitivity to variability in
formerly inaccessible interior regions of the cluster. We find 23 RRab and 11
RRc stars within one cluster radius (2.4$\arcmin$), for which we provide
coordinates, finder-charts and time-series photometry. Through Fourier
decomposition of the RR Lyrae star light curves we derive a mean metallicity of
[Fe/H]$_{\mathrm{UVES}} = -1.13 \pm 0.06$ (${\rm [Fe/H]}_{\mathrm{ZW}} = -1.25
\pm 0.06$), and a distance of $d\approx 6.35 \pm 0.81$ kpc. Using the RR Lyrae
population, we also determine that NGC 6401 is an Oosterhoff type I cluster.
