  The unprecedented detail of the WFPC2 colour-magnitude diagrams of the
resolved stellar population of Leo A presented here allows us to determine a
new distance and an accurate star formation history for this extremely
metal-poor Local Group dwarf irregular galaxy. From the position of the red
clump, the helium-burning blue loops and the tip of the red giant branch, we
obtain a distance modulus, m-M=24.2+/-0.2, or 690 +/- 60 kpc, which places Leo
A firmly within the Local Group. Our interpretation of these features in the
WFPC2 CMDs at this new distance based upon extremely low metallicity (Z=0.0004)
theoretical stellar evolution models suggests that this galaxy is predominantly
young, i.e. <2 Gyr old. A major episode of star formation 900 - 1500 Gyr ago
can explain the red clump luminosity and also fits in with our interpretation
of the number of anomalous Cepheid variable stars seen in this galaxy. We
cannot rule out the presence of an older, underlying globular cluster age
stellar population with these data. However, using the currently available
stellar evolution models, it would appear that such an older population is
limited to no more than 10% of the total star formation to have occured in this
galaxy. Leo A provides a nearby laboratory for studying young metal poor stars
and investigations of metal-poor galaxy evolution, such as is supposed to occur
for larger systems at intermediate and high redshifts.
