Top panel: Optical image of the extremely-metal deficient (XMD) galaxy DDO 68, in g-filter, taken from the SDSS database. Letters A to F mark the regions, in which colours and ages of stellar population were estimated.

Bottom panel: Two-colour diagram (g-r) versus (u-g) with theoretical tracks for evolving stellar populations according to models from PEGASE2 package. The standard Salpeter IMF and the metallicity z=0.0004, the nearest to that of DDO 68, were accepted for model tracks. Two extreme star-formation (SF) laws are shown by dashed (instantaneous starburst) and solid (continuous star formation with constant rate) lines. Numbers along the tracks mark the respected ages (in Gyr) since the beginning of SF. The observed colours of selected regions (labeled with respective letters) are shown by empty squares with the respective error bars (+-1 sigma). The paper is accepted to Astronomy Letters (PAZh). See also arXive:astro-ph/0712.4007.

11 February 2008