3.1. Leo A
Leo A ( DDO 69, Leo III, UGC 5364) is a nearby gas-rich Magellanic dI galaxy. Analysis of the WFPC2 CMDs of the resolved stellar population (see Figure 4) resulted a new distance and an accurate SFH to be determined for this extremely metal-poor galaxy (Tolstoy et al. 1998). From the position of the RC, BLs and the tip of the RGB, a distance modulus, m-M = 24.2 ± 0.2, or 690 ± 60 kpc, was obtained which places Leo A firmly within the LG. The interpretation of 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 RC luminosity and also fits in with the interpretation of the number of anomalous Cepheid variable stars seen in this galaxy. The presence of an older, underlying globular cluster age stellar population could not be ruled out 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 the centre of this galaxy. Using theoretical models of the chemical evolution of dwarf galaxies (Ferrara & Tolstoy 1999) however, it is clear that even though this galaxy is extremely metal poor in order for it to build up the current metallicity it must contain a significant underlying older stellar population, perhaps in an extended outer halo of older stars. Of course neither the chemical evolution models nor the existing CMDs can distinguish between an old population which formed in a large burst, or more sedate and roughly constant rate through-out a longer time.