Radius-mass plane showing the size constraints on the companion star Roche lobe of MAXI J0911−655 (black solid line). The other curves represent theoretical mass-radius relations for hot (5 × 10⁶ K) and cold (10² K) pure He white dwarfs (red dotted lines), low-mass main sequence/brown dwarfs of age 5 Gyr (black dot-dashed lines) and 10 Gyr (cyan dashed line) for solar and sub-solar metallicity abundances. Top x-axes represents the corresponding binary inclination angle in degrees assuming a NS of 1.4 M_⊙. The hatched region represents the constraints on the companion mass from the binary mass function, while the blue shaded area represents the mass constraints for inclination angles between 75 and 90 degrees.