Variability and star formation in Leo T, the lowest luminosity
  star-forming galaxy known today
  We present results from the first combined study of variable stars and star
formation history (SFH) of the Milky Way (MW) "ultra-faint" dwarf (UFD) galaxy
Leo T, based on F606W and F814W multi-epoch archive observations obtained with
the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. We have
detected 14 variable stars in the galaxy. They include one fundamental-mode RR
Lyrae star and 10 Anomalous Cepheids with periods shorter than 1 day, thus
suggesting the occurrence of multiple star formation episodes in this UFD, of
which one about 10 Gyr ago produced the RR Lyrae star. A new estimate of the
distance to Leo T of 409 $^{+29}_{-27}$ kpc (distance modulus of 23.06 $\pm$
0.15 mag) was derived from the galaxy's RR Lyrae star. Our V, V-I
color-magnitude diagram of Leo T reaches V~29 mag and shows features typical of
a galaxy in transition between dwarf irregular and dwarf spheroidal types. A
quantitative analysis of the star formation history, based on the comparison of
the observed V,V-I CMD with the expected distribution of stars for different
evolutionary scenarios, confirms that Leo T has a complex star formation
history dominated by two enhanced periods about 1.5 and 9 Gyr ago,
respectively. The distribution of stars and gas shows that the galaxy has a
fairly asymmetric structure.
