Stellar parameters compared to a 13.5 Gyr isochrone constructed from the T6.0 diffusion model. Helium diffusion corrections to ﹩\mathrm{log}\,g﹩ have been applied to the TOP and SGB stars (+0.05 dex in both cases). The TOP stars have effective temperatures about 100 K lower than the turnoff point, in line with expectations from photometry. The cluster may be somewhat younger than 13.5 Gyr, which, however, has only a small effect on our comparison between observed abundance trends and atomic diffusion predictions.