  We theoretically analyze the temperature behavior of paraxial light in
thermal equilibrium with a dye-filled optical microcavity. At low temperatures
the photon gas undergoes Bose-Einstein condensation (BEC), and the photon
number in the cavity ground state becomes macroscopic with respect to the total
photon number. Owing to a grandcanonical excitation exchange between the photon
gas and the dye molecule reservoir, a regime with unusually large fluctuations
of the condensate number is predicted for this system that is not observed in
present atomic physics BEC experiments.
