  We have obtained [Mg/Fe] for around 77% of the stars of the MILES library of
stellar spectra in order to include this important information into simple
stellar population (SSP) models. The abundance ratios, which were carefully
calibrated to a single uniform scale, were obtained through a compilation from
high spectral resolution works plus robust spectroscopic analysis at medium
resolution. The high resolution data provided an extensive control sample.
Average uncertainties (0.06 and 0.12 dex for the high and medium resolution
samples respectively) and the good coverage of the stars with [Mg/Fe] over the
MILES's parameter space will permit us to semi-empirically build up new SSP
models with accurate alpha-enhancements for ages older than 1 Gyr. This will
open new prospects for evolutionary stellar population synthesis.
