Stellar Archaeology in the Galactic halo with Ultra-Faint Dwarfs: VII.
  Hercules
  We present the first time-series study of the ultra-faint dwarf galaxy
Hercules. Using a variety of telescope/instrument facilities we secured about
50 V and 80 B epochs. These data allowed us to detect and characterize 10
pulsating variable stars in Hercules. Our final sample includes 6
fundamental-mode (ab-type) and 3 first overtone (c-type) RR Lyrae stars, and
one Anomalous Cepheid. The average period of the ab-type RR Lyrae stars, < Pab
>= 0.68 d (sigma = 0.03 d), places Hercules in the Oosterhoff II group, as
found for almost the totality of the ultra-faint dwarf galaxies investigated so
far for variability. The RR Lyrae stars were used to obtain independent
estimates of the metallicity, reddening and distance to Hercules, for which we
find: [Fe/H] = -2.30+-0.15 dex, E(B -V) = 0.09+-0.02 mag, and (m-M)o =
20.6+-0.1 mag, in good agreement with the literature values. We have obtained a
V, B - V color-magnitude diagram (CMD) of Hercules that reaches V ~ 25 mag and
extends beyond the galaxy's half-light radius over a total area of 40' {\times}
36'. The CMD and the RR Lyrae stars indicate the presence of a population as
old and metal-poor as (at least) the Galactic globular clusters M68.
