RR Lyrae and short-period variable stars in the dwarf irregular galaxy NGC 6822

We report the discovery of a large number of short-period variable stars in the dwarf irregular galaxy NGC 6822, based on deep time-series imaging carried out with the ESO Very Large Telescope. In particular, we found a modest population of RR Lyrae stars tracing the presence of an old stellar component in NGC 6822. Measurements of the average luminosity of RR Lyrae stars provide a new independent estimate of the distance to this galaxy based on a Population II indicator, (m - M)(o) = 23.36 +/- 0.17. In addition, our new data show a significant population of small-amplitude, short-period variable stars filling the instability strip starting at luminosities only a few tenths of a magnitude brighter than the RR Lyrae stars. Given the presence of an extended star formation in NGC 6822, the faint end of this distribution of short-period variable stars is likely to originate from a population of intermediate-age, metal-poor He-burning stars, younger and more massive than RR Lyrae stars.