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 , \langle P _ { ab } \rangle =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 \pm 0.15 dex , E ( B - V ) = 0.09 \pm 0.02 mag , and ( m - M ) _ { 0 } = 20.6 \pm 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 \sim 25 mag and extends beyond the galaxy ’ s half-light radius over a total area of 40 ^ { \prime } \times 36 ^ { \prime } .
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 .