  Using the near-infrared spectral stellar library of Cenarro et al. (2001a,b),
the behaviours of the Mg I line at 8807 angstrom and nearby TiO bands are
analyzed in terms of the effective temperature, surface gravity, and
metallicity of the library stars. New spectroscopic indices for both spectral
features -namely MgI and sTiO- are defined, and their sensitivities to
different signal-to-noise ratios, spectral resolutions, flux calibrations, and
sky emission line residuals are characterized. The new two indices exhibit
interesting properties. In particular, MgI reveals as a good indicator of the
Mg abundance, whereas sTiO is a powerful dwarf-to-giant discriminator for cold
spectral types. Empirical fitting polynomials that reproduce the strength of
the new indices as a function of the stellar atmospheric parameters are
computed, and a FORTRAN routine with the fitting functions predictions is made
available. A thorough study of several error sources, non-solar [Mg/Fe] ratios,
and their influence on the fitting function residuals is also presented. From
this analysis, a [Mg/Fe] underabundance of ~ -0.04 is derived for the Galactic
open cluster M67.
