  We explore the potential of the CERN Axion Solar Telescope (CAST) for testing
the presence of large extra dimensions. The CAST experiment has originally been
proposed to search for solar axions with a sensitivity supposed to provide a
limit on the axion-photon coupling g_{a\gamma\gamma}<5x10^{-11} GeV^{-1} or
even lower. The expected bound on the coupling constant is by a factor of ten
more stringent than the current experimental results. This bound extends for
the first time beyond the limit dictated by astrophysical considerations. As a
tuning experiment planning to explore the axion mass region up to about 1 eV,
CAST would also be sensitive to the existence of Kaluza-Klein massive states.
Therefore, the detection of X-rays at least at two pressures may be the
signature of large extra dimensions. From this requirement we find that CAST
may test (two) large extra dimensions with a (common) compactification radius R
down to around 250 nm if m_{PQ}<1/(2R), and down to around 370 nm if
1/(2R)<m_{PQ}, where m_{PQ} is the Peccei-Quinn mass.
