Membership selection for stars observed in NGC 6569. The upper panel shows the heliocentric radial velocity plotted against V − V_HB. The dotted line is the cluster velocity given by the Harris catalogue, the dashed line is the mean velocity for the 7 stars ultimately selected as cluster members. The stars unambiguously identified as radial velocity non-members are plotted as  × -signs. The lower panel shows the sum of the strengths of the λ8542 and λ8662 lines plotted against V − V_HB. Here, 4 stars, plotted as open circles, have line strengths incompatible with the remaining 7, plotted as filled circles, which have a small dispersion about a line of slope –0.627 Å mag^-1 (see Sect. 4). These 4 stars are classified as line-strength non-members.