Title: Telling star clusters from galaxies in the `ultra-compact dwarf'' regime

Abstract:
It seems established that ultra-compact dwarf galaxies (UCDs) are a mixture of massive 
globular clusters, stripped nuclear star clusters and extend, without a clear gap, into the regime 
of compact ellipticals. But is it possible to pin-point a likely origin, i.e. compact galaxy, or star 
cluster, to individual objects?
Galactic and extra-galactic star clusters show an enhancement in nitrogen uniquely linked to 
the `second generation'' stars formed in the high-density, short time-scale `cluster'' mode of star 
formation. Early-type (compact or dwarf) galaxies, on the other hand, show no such 
enhancement. Analysing the CN absorption bands in the integrated spectrum of individual 
UCDs, we find evidence that UCDs indeed appear to inhabit both, the CN-enhanced cluster, and 
the CN-normal galaxy populations. Thus, the measurement of the CN (or nitrogen) abundance 
provides a powerful tool to separate UCDs into star clusters and compact galaxies and may be 
our best chance in tracing the cluster-dwarf galaxy interface on the `compact'' side.