  String theory compactifications may generate many light axion-like particles
(ALPs) with weak couplings to electromagnetism. In general, a large number of
ALPs may exist, with a linear combination having a potentially observable
coupling to electromagnetism. The basis in which only one ALP couples to
electromagnetism is in general misaligned with the mass basis. This leads to
mixing between the `electromagnetic' ALP and a number of `hidden' ALPs that do
not interact directly with the photon. The process is analagous to neutrino
oscillations. I will discuss the phenomenological consequences of this mixing
for astrophysical ALP signals, in particular showing that it may significantly
reduce the predicted signal in experiments such as the CERN Axion Solar
Telescope.
