Top panel: stellar surface metallicity (normalized by the initial metallicity) after a metal-free accretion during the last 0.03 M_final as a function of stellar mass. The solid and dashed lines denote the cases with X_D = 28 and 20 ppm, respectively, and ξ = 0.5, 0.1 and 0 from top to bottom. In the ξ = 0.5 case, both lines are overlapped. Bottom panel: surface metallicity as a function of the stellar effective temperature, T_eff, in the case with ξ = 0.1 and X_D = 28 ppm (the green dashed line in the top panel). The same points are used as in the top panel.