An HR diagram in the plane (logT(eff), M-bol) is presented for population II subdwarfs and subgiants having Hipparcos parallaxes known with an accuracy better than 15 per cent. The effective temperatures and the apparent bolometric magnitudes have been taken from measurements made by Alonso et al. (1996), by the IRFM method for the effective temperatures, and direct integration of the fluxes for the bolometric fluxes. This has allowed to bypass the use of bolometric corrections, and the resulting errors. The resulting diagram is interpreted with two sets of isochrones computed with OPAL opacities enhanced in alpha-elements, updated equations of state, a helium abundance close to the primordial value, and a mixing length to pressure scale height ratio calibrated on the Sun. For the first time it has been possible to check the validity of theoretical models for metal-poor stars against accurate observations, and to derive the age of halo stars independently of any globular cluster data.
