  Hawking evaporation of photons in a Vaidya-de Sitter black hole is
investigated by using the method of generalized tortoise coordinate
transformation. Both the location and the temperature of the event horizon
depend on the time. It is shown that Hawking radiation of photons exists only
for the complex Maxwell scalar $\phi_0$ in the advanced Eddington-Finkelstein
coordinate system. This asymmetry of Hawking radiation for different components
of Maxwell fields probably arises from the asymmetry of spacetime in the
advanced Eddington-Finkelstein coordinate system. It is shown that the black
body radiant spectrum of photons resembles that of Klein-Gordon particles.
  PACS numbers: 04.70.Dy, 97.60.Lf

General Relativity and Quantum Cosmology