Orbital period evolution for LMXB systems with a donor mass of 1.4 M_⊙, a NS mass of 1.3 M_⊙ and a magnetic braking index of γ = 5. The black circles represent the onset of RLO and the red stars show the end of the mass-transfer phase. The observed MSP systems mainly investigated here, with low-mass He WD companions and P_orb = 2−9h (cf. Table 1), are located within the grey shaded region. The LMXB systems experience a second RLO and evolve into ultra-compact X-ray binaries in very tight orbits.