  We present Gran-Telescopio-Canarias/OSIRIS optical spectra of 4 of the most
compact and massive early-type galaxies in the Groth Strip Survey at redshift
z~1, with effective radii Reff=0.5-2.4 kpc and photometric stellar masses
Mstar=1.2-4x10^11 Msun. We find these galaxies have velocity dispersions
sigma=156-236 km/s. The spectra are well fitted by single stellar population
models with approximately 1 Gyr of age and solar metallicity. We find that: i)
the dynamical masses of these galaxies are systematically smaller by a factor
of ~6 than the published stellar masses using BRIJK photometry; ii) when
estimating stellar masses as 0.7xMdyn, a combination of passive luminosity
fading with mass/size growth due to minor mergers can plausibly evolve our
objects to match the properties of the local population of early-type galaxies.
