Fig. 1: Constraints on the mass and radius of the companion star.

a, A generic constraint on the companion mass as a function of inclination angle, i, on the basis of the measured mass function for different pulsar masses of 1.0, 1.4 and 2.0 M_⊙. If the orbital plane is randomly distributed, the integrated probability of inclination angle i from 90^o (edge-on) to 0^o (face-on) can be calculated as cos(i). The observed lower limit of a white dwarf (WD) mass is 0.16 M_⊙(light-blue dashed–dotted horizontal line). The probability of the companion being a white dwarf is smaller than 0.3%. b, Constraints on the mass and radius of the companion star. The red and green regions denote the mass–radius relations of the companion star at solar (LMX model) and zero (LMZ model) metallicities²³, respectively, with an evolution age of between 3 and 10 Gyr. Assuming a pulsar mass of 1.4 M_⊙ (M_p), the radius of the Roche lobe as a function of companion mass is plotted as the black curve, which serves as the upper limit of the allowed parameter space. The dotted regions are the allowed regions for the companion mass and radius.