Zero-temperature white dwarf mass-radius relations by Eggleton (Rappaport et al. 1987) for pure helium, carbon and oxygen compositions (dashed), and the present Roche-lobe radius against mass for the companion of PSR J1719–1438. Filled circles (neutron star mass of 1.4  M_⊙) and open circles (neutron star mass of 2  M_⊙) from low to high companion mass indicate the 1, 0.1 and 0.01 a priori probabilities for a companion mass higher than indicated.