Left panel. SDSS g − r-colours plotted against u − g of all stars. The grey dots mark all stellar objects with spectra available in the SDSS database. Most of them are classified as DA white dwarfs. The solid diamonds mark (He-)sdO stars, the solid squares sdB and sdOB stars. Open squares mark hot subdwarfs with main sequence companions visible in the spectra. Most of these objects are white dwarfs of DA type. Right panel. Only subdwarfs with g < 18 mag are plotted. The sequence of composite objects is clearly separated from the single-lined stars. Synthetic colours from Castelli & Kurucz (2003) for stars with temperatures ranging from 14 000 K to 50 000 K (log g = 5.0) are marked with upward triangles and connected. The stepsize of the colour grid is 1000 K. The labels mark models of certain temperatures.