a result of using single-burst equivalent ages. Single-burst equivalent ages effectively measure a luminosity-weighted age. Therefore, even a relatively minor episode of star formation is sufficient to significantly underestimate the age of the bulk of (old) stars. Indeed, many of the galaxies in the Treu et al. sample show \[\] $\lambda$3727 emission, consistent with minor ongoing star formation activity [@TSCMB02]. Is this contamination enough to bias our results, or is the use of the relative ages of the old-envelope sufficient to reduce this bias to useful levels?

To answer this question we consider a scenario where the bulk of stars in E/S0 galaxies formed at high redshift, while a relatively small fraction (up to 10% in mass) of stars is formed at later times ($z<1.5$). This scenario is approximately consistent with the observed evolution of the number density [@Im+02], colors [@JFDTPN99; @MAE01], and spectral properties of early-type galaxies [@Trager+00; @TSCMB02]. Within this scenario, we use Monte Carlo simulations to estimate the effects of this bias on the age–redshift relation and ultimately on $H_0$ and $w(z)$ determinations. We generate galaxy spectra assuming that after the initial burst of star formation at $z>3$ an extra 10% of the stellar mass is formed 