Colour–colour plot of K −  [5.8]  versus  [8]  −  [24] . The small grey dots in the background are the sources of the SAGE catalogue, the black crosses represent the 11 RV Tauri stars in the LMC from the OGLE-III catalogue (Soszyński et al. 2008) that are detected at 8 and 24 μm and the black dots and squares are the Galactic disc sources from De Ruyter et al. (2006) and some Galactic shell sources observed with the IR spectrometer SWS onboard ISO transformed onto the Spitzer filters. The black solid line indicates the colour-cut we are using to select likely post-AGB candidates, the lower dashed line indicates the lower limit for post-AGB candidates with a shell and the upper dashed line the upper limit for post-AGB candidates with a disc (see Sect. 2.5).