Null geodesics in black hole metrics with non-zero cosmological constant

We study the radial motion along null geodesics in the Reissner-Nordström-de Sitter and Kerr-de Sitter space-times. We analyze the properties of the effective potential and we discuss circular orbits. We find that:

1)

the radius of circular photon orbits in the Reissner-Nordström-de Sitter space-times does not depend on the cosmological constant. We show also how this is related to properties of the optical reference geometry.

2)

For a specific range of the cosmological constant, photons with high impact parameter may travel radially between the cosmological horizon and the black hole horizon in the equatorial plane of the Kerr-de Sitter space-times.