  We show that the rest mass of a system consisting of two photons is a
relativistic invariant having the same magnitude in all inertial reference
frames in relative motion. A scenario which starts with two photons where
theirs frequencies are equal to each other the magnitude of the rest mass of
the system depends on the angle made by the momentums of the two photons. The
behavior of the photons when detected from two inertial reference frames is
illustrated using a relativistic diagram which displays in true values the
physical quantities (scalar and vector) introduced in order to characterize the
dynamical properties of the photon. We consider that the obtained results
should be taken in the discussions, some time fierce, between those who ban the
concept of relativistic mass and those who consider that its use is harmless.
