  We have extended the study of the young star clusters observed in the merger
remnant galaxy NGC 7252 by obtaining K band photometry for these clusters. Our
K band data significantly complement the optical photometry and spectroscopy in
the literature: K band data are fundamental to study the Asymptotic Giant
Branch (AGB) population of these clusters, since the AGB phase transition
(occuring between the age of 200 Myr and 1 Gyr) causes abrupt changes in the
near-infrared luminosity of the clusters while producing only small changes in
the optical. Therefore, the e.g. V-K colour is ideal to study this evolutionary
phase of stellar populations. For the present analysis we present models for
Simple Stellar Populations which include the contribution of the AGB stellar
phase, calibrated with the young and intermediate age star clusters of the
Magellanic Clouds. The comparison with the colour distribution of the NGC 7252
star clusters shows that they are indeed intermediate age clusters undergoing
the AGB phase transition. The AGB phase transition is observed for the first
time outside the Local Group. Most of the studied clusters span the very narrow
age range 300-500 Myr, and likely have metallicities 0.5-1 Z_{\odot}. A very
important exception is the cluster W32, which has already completed its AGB
epoch, its colours being consistent with an age of 1-2 Gyr. This impacts on the
duration of the merger-induced starburst. The strengths of the magnesium and
iron lines in the spectrum of the best observed cluster W3, and in the spectrum
of the diffuse central light of NGC 7252, do not show an overabundance in
alpha-elements, in contrast to the bulk stellar population of elliptical
galaxies.
