  During 2003--2015, the CERN Axion Solar Telescope (CAST) has searched for
$a\to\gamma$ conversion in the 9 T magnetic field of a refurbished LHC test
magnet that can be directed toward the Sun. In its final phase of solar axion
searches (2013--2015), CAST has returned to evacuated magnet pipes, which is
optimal for small axion masses. The absence of a significant signal above
background provides a world leading limit of $g_{a\gamma} < 0.66 \times
10^{-10} {\rm GeV}^{-1}$ (95% C.L.) on the axion-photon coupling strength for
$m_a \lesssim 0.02$ eV. Compared with the first vacuum phase (2003--2004), the
sensitivity was vastly increased with low-background x-ray detectors and a new
x-ray telescope. These innovations also serve as pathfinders for a possible
next-generation axion helioscope.
