The final stage of mass transfer in a compact binary system is very important for calculating the orbital parameters of the resulting system. We demonstrate how simple ‘bifurcation mechanisms‘ may lead to the existence of three classes of binary millisecond pulsars and a possible gap in the orbital period distribution. Recent discoveries indicate that such a gap could exist (see Fig. 2).

Furthermore, we demonstrate that the expected theoretical correlation (e.g. Joss et al. 1987, Rappaport et al. 1995) between orbital periods and white dwarf companion masses does not seem to fit observations of wide-orbit lowmass binary pulsars (LMBPs). It is interesting to notice that the five LMBPs with Porb > 100d all have companion masses which are lighter (at the ∼80 % confidence level on average) than expected from the theoretical core-mass period relation.