Donor mass versus orbital period for UCXBs. The solid curves represent a zero-temperature carbon-oxygen (gray) and helium (black) white dwarf donor. The dashed curves are different from the solid curves only in that they have a twice as large donor radius at all masses. The circles on top of the solid and dashed curves mark the age of the system since the onset of mass transfer, the numbers associated with the circles indicate the ages in yr. Filled (open) symbols indicate an initial accretor mass of 1.4  M_⊙  (2  M_⊙). The dotted extensions to the curves go beyond the point that a UCXB with a 1.4  M_⊙ accretor can reach within the age of the Universe. The vertical solid line shows the present orbital period of PSR J1719–1438. The overlaying triangles give the a priori probabilities for the donor mass being higher than indicated, based on the mass function.