OB associations have a rich population of massive stars (O and B stars), which are young and typically dispersed over large regions of the sky. While these massive stars can be easily recognized thanks to their intense luminosity, it is very hard to select the low mass stars belonging to OB associations. This because without evident stellar concentrations, these stars
OB associations are objects of great importance, since they contain a rich population of OB stars. Despite their paucity, OB stars have a strong impact on the evolution of their parental clouds, nearby stars, and the whole Galaxy. For instance, their intense UV emission affects the evolution of the parental cloud and the star formation process in it, sometimes triggering
