Upper panel: spin-up tracks of accreting NSs leading to presently observed PSR J1903+0327 parameters on radius-spin frequency plane. The relevant characteristic radii are: stellar radius (solid curves), the radius of a marginally stable orbit (dotted curves), and the radius of the inner boundary of accretion disk (dashed curves). Colors correspond to the specific values of accretion rate and the initial value of the magnetic field. Lower panel: effective radial potential V(r,l(r)) in the energy (mc²) units for an exact, approximate, and Schwarzschild solution; the dot marks the final r₀. See the text for more details (color online).