Left panel: concentration index in the r band, C_r, versus the D_n(4000) index. The vertical lines correspond to the two definitions of ETGs (C_r> 2.86 or 2.60, see text for details), while the horizontal line separates blue from red sources. Right panel: D_n(4000) versus L_{1.4 GHz}/M_∗. The dashed curve is the empirical separation between the star-forming and AGN radio-emitting sources performed by Best et al. (2005; read the text for details). The dotted curve is shifted by 0.1 above in D_n(4000) of the dashed curve to separate the objects with a possible star-formation contribution to their radio emission. The small dot points correspond to the SDSS/NVSS sample studied by Baldi & Capetti (2010). The different color/shape symbols correspond to the different classes like in Fig. 1.