Best fit model for the third population in the OB-I diagram. The grey contours represent the FADO-SDSS sample; the green points are the NII-Composite galaxies; the black line is our separation line given by Eq. (2); the black symbols represent the 0.2 Z_⊙ model for all scenarios; the red symbols represent the 0.4 Z_⊙ model for all scenarios; the cyan symbols represent the Z_⊙ model for all scenarios. The red dot-dashed line represents the best fit for all the points in the diagram, given by Eq. (3). The grey shaded area represents the 1σ uncertainty associated with the fit.