Maximum density of OB stars projected on the galactic plane towards Orion. The positions of Orion OB1a,b,c, and d and Platais et al. (1998) candidate associations and clusters are indicated with squares. The colour scale is proportional to their age when an estimate is available in the literature and transparent otherwise. The giants and supergiants composing the constellation asterism are represented by light blue stars. This should be read jointly with Fig. 1.