The log Ṁ_tr – P_orb plane. Dots – persistent AM CVn systems with known distances; asterisks – outbursting AM CVn systems with known distances; solid lines – the upper and lower critical limits Ṁ_tr: (1) for M₁ = 1.0, Y = 1.0; (2) for M₁ = 0.6, Y = 1.0; (3) for M₁ = 1.0, Y = 0.96,Z = 0.04; (4) for M₁ = 1.0, Y = 1.0; (5) for M₁ = 0.6, Y = 1.0. (M₁ – in solar units). Dotted lines: evolution models for AM CVns through the WD channel (kindly provided by Chris Deloye).