  We have used precise radial velocity measurements for a large number of
candidate low-mass stars and brown dwarfs, to show that the young Sigma Ori
``cluster'' consists of two spatially superimposed components which are
kinematically separated by 7 km/s in radial velocity, and which have different
mean ages. We examine the relationship of these two kinematic groups to other
populations in the Orion OB1 association and briefly discuss the consequence of
mixed age samples for ongoing investigations of the formation and evolution of
low-mass objects in this much-observed region.
