This paper is grounded exclusively on the GR-Hamilton’s variational principle,reducible to the principle of geodesics in the case of a body in free motion in agravitational field. The first two sections deal with the theory of gravitational lightbending, available in all general relativity texts, but rewritten here using exclu-sively formulas between quantities, more suitable for the physical interpretationthan the formulas between measurements commonly used (c = 1, G = 1).The third section deals the main question: what properties should GR assignto a particle moving according to a null-geodesics? Without resorting to anyprevious theory, the general relativity itself imposes very stringent and interestingproperties for "photons" (of course in agreement with those asserted by specialrelativity in the absence of a gravitational field).