Photometric Identification of the low-Mass Population of Orion OB1b I:   The sigma Ori Cluster
  We report an optical photometric survey of 0.89 sq. degrees of the Orion OB1b association centered on sigma Ori. This region includes most of the sigma Ori cluster, the highest density region within Orion OB1b. We have developed a statistical procedure to identify the young, low-mass pre-main sequence population of the association. We estimate that the cluster has ~160 members in the mass range (0.2<M<1Msun). The cluster has a radius of ~3.5 pc and an estimated age of 2.5+/-0.3 Myr. We estimate that the total mass of the cluster is 225+/-30 Msun. This mass is similar to the estimated mass of the ~0.5 Myr old cluster NGC 2024. NGC 2024 and sigma Ori appear to be a well matched pair of clusters, except for the ~2 Myr difference in their ages. 
W. H. Sherry, F. M. Walter, S. J. Wolk