Semi-major axis as a function of the eccentricity for a precession of the periastron with a period of 70 days. The parameters are those infered for PSR 1931 + 24, and the relation is derived from Eq. (1). The two other curves represent the periastron and the apoastron. The semi-major axis a is given in units of the light cylinder radius R_lc. The values of the semi-major axis compatible with both on and off modes correspond to the part of the curve on the left-hand side of the vertical line e = e_max. This defines a rather narrow domain of a possible semi-major axis: 0.9 × 10⁸ < a < 1. × 10⁸.