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We present the first investigation of the Blue Straggler star (BSS)population in two isolated dwarf spheroidal galaxies of the Local Group, Cetusand Tucana. Deep HST/ACS photometry allowed us to identify samples of 940 and1214 candidates, respectively. The analysis of the star formation histories ofthe two galaxies suggests that both host a population of BSSs. Specifically, ifthe BSS candidates are interpreted as young main sequence stars, they do notconform to their galaxy's age-metallicity relationship. The analysis of theluminosity function and the radial distributions support this conclusion, andsuggest a non-collisional mechanism for the BSS formation, from the evolutionof primordial binaries. This scenario is also supported by the results of newdynamical simulations presented here. Both galaxies coincide with therelationship between the BSS frequency and the absolute visual magnitude Mvfound by Momany et al (2007). If this relationship is confirmed by largersample, then it could be a valuable tool to discriminate between the presenceof BSSs and galaxies hosting truly young populations.

The ACS LCID project VII: the blue stragglers population in the isolated dSph galaxies Cetus and Tucana