We investigate on the relative inclination of the planets B and C orbiting the pulsar PSR B1257+12 .
First , we show that the third Kepler law does represent an adequate model for the orbital periods P of the planets , because other Newtonian and Einsteinian corrections are orders of magnitude smaller than the accuracy in measuring P _ { B / C } .
Then , on the basis of available timing data , we determine the ratio \sin i _ { C } / \sin i _ { B } = 0.92 \pm 0.05 of the orbital inclinations i _ { B } and i _ { C } independently of the pulsar ’ s mass M .
It turns out that coplanarity of the orbits of B and C would imply a violation of the equivalence principle .
Adopting a pulsar mass range 1 \lesssim M \lesssim 3 , in solar masses ( supported by present-day theoretical and observational bounds for pulsar ’ s masses ) , both face-on and edge-on orbital configurations for the orbits of the two planets are ruled out ; the acceptable inclinations for B span the range 36 deg \lesssim i _ { B } \lesssim 66 deg , with a corresponding relative inclination range 6 deg \lesssim ( i _ { C } - i _ { B } ) \lesssim 13 deg .