  Near-infrared (hereafter NIR) data may provide complementary information to
the traditional optical population synthesis analysis of unresolved stellar
populations because the spectral energy distribution of the galaxies in the
1-2.5\mum range is dominated by different types of stars than at optical
wavelengths. Furthermore, NIR data are subjected to less absorption and hence
could constrain the stellar populations in dust-obscured galaxies. We want to
develop observational constraints on the stellar populations of unresolved
stellar systems in the NIR. To achieve this goal we need a benchmark sample of
NIR spectra of ``simple'' early-type galaxies, to be used for testing and
calibrating the outputs of population synthesis models. We obtained
low-resolution (R~1000) long-slit spectra between 1.5 and 2.4\mum for 14 nearby
early-type galaxies using SofI at NTT and higher resolution (R~3000) long-slit
spectra, centered at the MgI at ~1.51\mum for a heterogeneous sample of 5
nearby galaxies observed with ISAAC at VLT. We defined spectral indices for CO,
NaI, CaI and MgI features and measured the strengths of these features in the
sample galaxies. We defined a new global NIR metallicity index, suitable for
abundance measurements in low-resolution spectra. Finally, we present an
average NIR spectrum of an early-type galaxy, built from a homogenized subset
of our sample. The NIR spectra of the sample galaxies show great similarity and
the strength of some features does correlate with the iron abundance [Fe/H] and
optical metal features of the galaxies. The data suggest that the NIR metal
features, in combination with a hydrogen absorption feature may be able to
break the age-metallicity degeneracy just like the Mg and Fe features in the
optical wavelength range.
