Stellar populations and Star Formation History of the Metal-Poor Dwarf
  Galaxy DDO 68
  We present the star formation history of the extremely metal-poor dwarf
galaxy DDO 68, based on our photometry with the Advanced Camera for Surveys.
With a metallicity of only $12+\log(O/H)=7.15$ and a very isolated location,
DDO 68 is one of the most metal-poor galaxies known. It has been argued that
DDO 68 is a young system that started forming stars only $\sim 0.15$ Gyr ago.
Our data provide a deep and uncontaminated optical color-magnitude diagram that
allows us to disprove this hypothesis, since we find a population of at least
$\sim 1$ Gyr old stars. The star formation activity has been fairly continuous
over all the look-back time. The current rate is quite low, and the highest
activity occurred between 10 and 100 Myr ago. The average star formation rate
over the whole Hubble time is $\simeq 0.01$ M$_{\odot}$ yr$^{-1}$,
corresponding to a total astrated mass of $\simeq 1.3 \times 10^8$ M$_{\odot}$.
Our photometry allows us to infer the distance from the tip of the red giant
branch, $D = 12.08 \pm 0.67$ Mpc; however, to let our synthetic color-magnitude
diagram reproduce the observed ones we need a slightly higher distance,
$D=12.65$ Mpc, or $(m-M)_0 = 30.51$, still inside the errors of the previous
determination, and we adopt the latter. DDO 68 shows a very interesting and
complex history, with its quite disturbed shape and a long Tail probably due to
tidal interactions. The star formation history of the Tail differs from that of
the main body mainly for an enhanced activity at recent epochs, likely
triggered by the interaction.
