Evolution of [Fe/H] (upper panels), [C/H] (middle panels) and [C/Fe] (lower panels) in CEMP-s models of 0.8 M_⊙ (left panels) and 0.85 M_⊙ (right panels) with diffusion. Thick lines are models with radiative levitation, whereas thin lines are those without. Empty circles mark an age of 10 Gyr, and filled circles mark an age of 13.8 Gyr, i.e. the part of the track between the circles covers the expected age range of CEMP-s stars. The small, grey circles show the metal-poor stars observed in the Sloan Digital Sky Survey in the metallicity range − 2.5 ≤ [Fe/H] ≤ −2.0 (Lee et al. 2013).