Radius-mass plane showing the size constraints on the companion star Roche-lobe of IGR J17591–2342 (black solid line) obtained from the orbital parameters of the neutron star. The hatched region represents the constraints on the companion mass from the binary mass function, and the blue area defines the mass constraints for inclination angles between 60 and 90°. The other curves represent theoretical mass-radius relations for zero-age main-sequence stars (purple crosses) and isochrones for stars of 8 (red diamonds) and 12 Gyr (blue circles). The top x-axes represent the corresponding binary inclination angle in degrees assuming a 1.4 M_⊙.