  We have obtained [Mg/Fe] measurements for 76.3% of the stars in the MILES
spectral library used for understanding stellar atmospheres and stellar
populations in galaxies and star clusters. These abundance ratios were obtained
through (1) a compilation of values from the literature using abundances from
high-resolution spectroscopic studies and (2) a robust spectroscopic analysis
using the MILES mid-resolution optical spectra. All the [Mg/Fe] values were
carefully calibrated to a single uniform scale, by using an extensive control
sample with results from high-resolution spectra. The small average
uncertainties in the calibrated [Mg/Fe] values (respectively 0.09 and 0.12 dex
with methods (1) and (2)) and the good coverage of the stars with [Mg/Fe] over
stellar atmospheric parameter space of the library will permit the building of
new simple stellar populations (SSPs) with empirical $\alpha$-enhancements.
These will be available for a range of [Mg/Fe], including both sub-solar and
super-solar values, and for several metallicities and ages. These models will
open up new prospects for testing and applications of evolutionary stellar
population synthesis.
