Characteristic radii of XB 1916-053 in units of orbital separation vs the mass ratio q. Show are the NS Roche-lobe radius (blue curve), the radius of the disk at which a 2:1 and 3:1 resonance could occur (yellow and black curves), the radius associated with the apsidal rate of the disk for a 3:1 resonance without and with the pressure term due to the presence of a spiral wave in the disk (green and brown curve), and the radius associated with the nodal precession period for a 3:1 resonance (red curve).