T_eff versus EW(NaI) for members and non-members of Collinder 69. Rejected candidates are displayed as crosses, confirmed members as solid dots, where red ones highlight sources classified as active accretors and/or detected in our X-ray survey. Large open circles and squares surround sources showing infrared excess or classified as binaries in either Sacco et al. (2008) or Maxted et al. (2008). For those sources showing large variability in the measured Na I equivalent widths (measurements from Maxted et al. 2008 and this work), we have included vertical bars connecting those measurements. We note the unusual position of LOri115 (discussed in Sect. 5.3).