significant level.

Before we can use the estimated ages to constrain $H_0$ we have to estimate the effects of the potential systematic uncertainties related to adopting a single-burst stellar population model and a single-metallicity model. As far as the first uncertainty is concerned, we know that even massive field E/S0 galaxies can experience some level of star formation activity at $z>0.2$ [@MAE01; @TSCMB02; @WHWL02]. The presence of a young and luminous stellar population superimposed on a dominant old stellar population can bias the single-burst equivalent ages of the spectrum to be much younger than the mass-weighted average age. Although this is a serious concern for the interpretation of individual cases, our method relies on the old envelope of the age distribution at any given redshift. Therefore, the effects of this bias on the determination of $H_0$ and $w$ are strongly reduced, as we discuss further in § 3.2. Concerning single metallicity, the uncertainty connected to metallicity gradients is negligible with respect to other sources of uncertainty. In fact, our spectra typically cover $6$ kpc (for the LRG) to $10$ kpc (for the higher redshift samples). Given that typical metallicity gradients in E/S0s follow $d {\rm log} [{\rm Fe}/{\rm H}]/
d 