relation defined by the old envelope (§3.3) and derive an independent measurement of $H_0$ (§3.4). We show that at the level of accuracy of our measurement, the use of the age–redshift relation allows us to apply this method even if a fraction of the stars in E/S0 galaxies is assembled at $z < 1$ [e.g., @TSCMB02]. We also briefly discuss the prospects of using this method to derive $w(z)$.

Sample and measurement
----------------------

In order to apply this method we need high-quality spectra of a sample of old stellar populations covering the largest possible range in redshift. To this aim we combine the following data sets: (i) the luminous red galaxy (LRG) sample from the Sloan Digital Sky Survey (SDSS) early data release [@Eisenstein+01]; (ii) the sample of field early-type galaxies from @TSCMB99 [@TSMCB01; @TSCMB02 hereafter the Treu et al. sample]; (iii) a sample of red galaxies in the galaxy cluster MS1054$-$0321 at $z=0.83$; and (iv) the two radio galaxies 53W091 and 53W069 [@D+96; @Spinrad+97; @NDJH03 Dey et al., in preparation].

The LRG sample is the most heterogenous, since it includes red galaxies in general, regardless of morphology. The Treu et al.sample contains high quality spectra with average $S/N=16$ per 