Plot of the estimated luminosities from Sect. 2.3 versus the  [8]  −  [24]  colour. The displayed objects are the same as in Fig. 20. The vertical lines are the boundaries of the different subtypes of our post-AGB sample based on the shape of the SED, where the objects with a disc can be found between the two outer lines and those with a shell are to the right of the middle one. The dashed horizontal lines indicate the luminosity-cut we used.