  We consider some fundamental constants from the point of view of the duality
symmetry. Our analysis of duality is focused on three issues: the maximum
radiated power of gravitational waves, the cosmological constant, and the
magnetic monopole mass. We show that the maximum radiated power of
gravitational waves implies that the Planck time is a minimal time.
Furthermore, we prove that duality implies a quantization of the cosmological
constant. Finally, by using one of the Euler series for the number $\pi $, we
show that the Dirac electric-magnetic charge quantization implies a mass for
the magnetic monopole (or neutrino) of the order of $10^{-5}$ the mass of the
electron.
