Tracks for a helium white dwarf donor with an initially 1.4   M_⊙, 12 km radius neutron star (solid) and 10   M_⊙ black hole (dashed) accretor. The tracks start at the respective dynamical instability limits of 0.83   M_⊙ and 1.04   M_⊙, since systems can only survive if they start with a lower donor mass (Sect. 2.3). The isotropic re-emission limit is ignored in the first part of the track. The numbers correspond to the two circles on their right-hand side and represent the logarithm of the system age (yr), which is the time since the onset of mass transfer. The relative location of both tracks will be discussed in Sect. 4.2.2.