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 \sim 200 Myr and \sim 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 \sim 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 .