Radial abundance gradient for oxygen. The black solid line refers to the one infall model without threshold and with the inside-out formation (S1I) with metallicity dependent yields, the green long-dashed line corresponding to solar yields. The red short-dashed dotted line represents the model with a radial inflow of 1 km s^-1 (R1I1) with metallicity dependent yields and the blue long dashed line with solar yields. The data are mean values and relative errors from Cepheids by Andrievsky et al. (2002a,b).