Variable Stars in Local Group Galaxies. IV. RR Lyrae stars in the
  central regions of the low-density galaxy Crater II
  We present a search and analysis of variable stars in the recently discovered
Crater~II dwarf galaxy. Based on $B$, $V$, $I$ data collected with the Isaac
Newton Telescope (FoV$\sim$0.44 square degrees) we detected 37 variable stars,
of which 34 are bone-fide RR Lyrae stars of Crater~II (28 RRab, 4 RRc, 2 RRd).
We applied the metal-independent ($V$, $B-V$) Period--Wesenheit relation and
derived a true distance modulus ($\mu$ = 20.30$\pm$0.08 mag ($\sigma$=0.16
mag). Individual metallicities for RR Lyrae stars were derived by inversion of
the predicted $I$-band Period-Luminosity relation. We find a mean metallicity
of [Fe/H]=-1.64 and a standard deviation of $\sigma_{[Fe/H]}$ =0.21 dex,
compatible with either negligible or vanishing intrinsic metallicity
dispersion. The analysis of the Colour-Magnitude Diagram reveals a stark
paucity of blue horizontal branch stars, at odds with other Galactic dwarfs,
and globular clusters with similar metal abundances.
