Comparison of electron temperatures t_e(O iii) (t_e = 10^-4   T_e) obtained from [O iii]λ4363/(λ4959 + λ5007) and t_e(S iii) obtained from [S iii]λ6312/λ9069 emission-line ratios. VLT/UVES data are shown by stars, except for three objects with uncertain values, which are shown as open circles (see Sect. 3.1). Twenty-seven data points from SDSS DR3 (see text) are shown by red dots. Open blue diamonds, blue asterisks, and purple triangles correspond to data from Kehrig et al. (2006), Pérez-Montero & Diaz (2003) and García-Rojas & Esteban (2007), respectively. The dashed blue line in the upper panel connects locations of equal temperature. The thick blue lines (also in the upper panel) are the predicted t_e(S iii) – t_e(O iii) relations for H ii region models from Izotov et al. (2006). The dotted and thin solid black lines are the curves from Garnett (1992) and Pérez-Montero & Diaz (2005), respectively. Regression lines (solid lines) and 1σ alternatives (dashed lines) for all our data plus data from García-Rojas & Esteban (2007) are shown by purple lines. In the lower panel the error bars are shown for the VLT, SDSS, and García-Rojas & Esteban (2007) data. Additionally, regression lines for VLT+SDSS-only data are shown by black lines. (A color version of this figure is available in the online journal.)