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.
Comment: Accepted by A&A