Present oxygen abundance vs. gas fraction (upper panel) and Y vs. oxygen abundance (lower panel) as predicted by models without wind. All these models assume the same total infall mass M_inf = 10⁹ M_⊙. Big open black circles: models with different numbers of bursts; big open black triangles: models with different durations of bursts; big open black squares: models with different occurrence times of bursts; big open black pentacles: models with different SFE. Solid black, dotted red and dashed blue lines in the upper panel are the evolutionary tracks of model M9d4, M9tau and M9IMF. The solid black line in the lower panel is the best fit of all the model points. The observational data are the same as in Figs. 2 and 4, open cyan circles and open cyan triangles represent BCDs and dIrrs, respectively. The solid cyan line is the best fit of the observational data.