Logarithm of the effective temperature corresponding to the spectral class found by the FOSC as a function of the model effective temperature for reddened stars with E_B−V = 4. To test the method, the reddening was not let as a free parameter of the FOSC but was fixed to its actual value. Each symbol corresponds to a simulated star and their colour depends on value of the quality index, as indicated by the colour scale on the right-hand side of the plots. The solid black line is the exact correlation and the dotted black lines shows the ± 10% relative effective temperature band. Top: luminosity classes IV and V. Bottom: luminosity classes I, II, and III.