Star clusters or asterisms? 2MASS CMD and structural analyses of 15
  challenging targets
  (Abridged) Poorly-populated star clusters may have photometric and structural
properties not much different from asterisms, to the point that, in some cases,
widely-used databases present conflicting classifications. We investigate the
nature of a sample of challenging targets that have been classified either as
star clusters or asterisms in different studies. A few objects are studied for
the first time.
