  We have obtained deep color-magnitude diagrams of the Sagittarius dwarf
irregular galaxy with the HST/ACS. The new diagrams unveil for the first time
the star formation history of SagDIG.
  We identify a small population of red horizontal branch stars, which sets the
first epoch of star formation in SagDIG back to ~9-10 Gyr ago. This shows that
SagDIG has been able to maintain a very low mean metallicity (-2.2<[Fe/H]<-1.9)
over a 10 Gyr life time, and most importantly, that all Local Group dwarf
galaxies share an ancient (~10 Gyr) initial episode of star formation. Thus, at
the moment, IZw18 remains the only exception to this general trend, favoring
the so-called ``young galaxy'' scenario. However, a re-analysis of ACS
observations of IZw18 suggests that an older RGB population may be present also
in IZw18.
  We also compare the distribution of atomic hydrogen with that of stellar
populations of various ages. We find little correlation between stellar
populations older than ~1 Gyr and the HI. In particular we fail to find any
generation of stars that preferentially lie within the large HI hole. We
therefore exclude the possibility that the SagDIG ring-like HI distribution is
the result of multiple supernova events. Alternative scenarios have to be
explored.
