Size of the Roche lobe of the donor of IGR J17498−2921 (solid curve) and ZAMS mass-radius relation given by Chabrier & Baraffe (2000, dotted line). The masses and radii of donor stars filling their Roche lobe, calculated from the measured mass function assuming m₁ = 1.4 and a set of values of inclination, are indicated as black points. The left shaded region is excluded by the observed mass function and by the absence of eclipses (see text), while the right one is excluded by the condition that the companion star does not overfill its Roche lobe.