This graph allows the identification of contamination by quasars: previously known members of Collinder 69 from Dolan & Mathieu (1999) and Barrado y Navascués et al. (2004b, 2007) are displayed as green crosses and open circles (for Class III and II members, respectively). Quasars observed by the SWIRE team (Harvey et al. 2006) appear as gray open triangles. Counterparts to our XMM-Newton X-ray sources are shown as as black circles over-imposed by big red crosses (for quasars, rejected as cluster members based on this diagram) and as solid black circles (other sources, including possible cluster members and other contaminants). After Bouy et al. (2009).