Formation and evolution of a 0.242 Msun helium white dwarf in presence   of element diffusion
  A 0.242 Msun object that finally becomes a helium white dwarf is evolved from Roche lobe detachment down to very low luminosities. In doing so, we employ our stellar code to which we have added a set of routines that compute the effects due to gravitational settling, and chemical and thermal diffusion. Initial model is constructed by abstracting mass to a 1 Msun red giant branch model up to the moment at which the model begins to evolve bluewards. We find that element diffusion introduces noticeable changes in the internal structure of the star. In particular, models undergo three thermonuclear flashes instead of one flash as we found with the standard treatment. This fact has a large impact on the total mass fraction of hydrogen left in the star at entering the final cooling track. As a result, at late stages of evolution models with diffusion are characterized by a much smaller nuclear energy release, and they evolve significantly faster compared to those found with the standard treatment.   We find that models in which diffusion is considered predict evolutionary ages for the white dwarf companion to the millisecond pulsar PSR B1855+09 in good agreement with the spin-down age of the pulsar. 
Leandro Althaus, Aldo Serenelli and Omar Benvenuto