The ages and colours of cool helium‐core white dwarf stars

0.406, 0.360, 0.327, 0.292, 0.242, 0.196 and 0.169 M( and follow their evolution from the end of mass-loss episodes, during their pre-white dwarf evolution, down to very low surface luminosities. We find that when the effective temperature decreases below 4000 K, the emergent spectrum of these stars becomes bluer within time-scales of astrophysical interest. In particular, we analyse the evolution of our models in the colour ‐ colour and in the colour ‐ magnitude diagrams and find that helium-core white dwarfs with masses ranging from ,0.18 to 0.3 M( can reach the turn-off in their colours and become blue again within cooling times much less than 15 Gyr and then remain brighter than MV < 16:5. In view of these results, many low-mass helium white dwarfs could have had enough time to evolve to the domain of collision-induced absorption from molecular hydrogen, showing blue colours.