Phase portraits of the objects affected by secular libration islands. On the graph a), the object 135571 is the only one found to have a retrograde circulation of ω. On the graph b), the secular evolution of 2004 KZ₁₈ inside the mean-motion resonance makes ω oscillate around 0. On the graph c), there are two distinct regions according to the topology of the semi-secular phase-space, since the resonance of 2008 ST₂₉₁ is of type 1:k. Below the green line, the resonance island is doubled (as in Fig. 1c, d) resulting in asymmetric level curves. Above the green line, only one resonance island remains (as in Fig. 1a, b) and the level curves are symmetric. If the line is crossed, there can be either a discontinuity (thick green line) if the particle is located in the vanishing island, or a smooth transition (dotted green line) if it is located in the persisting island.