  Hypothetical axions provide a compelling explanation for dark matter and
could be emitted from the hot solar interior. The CERN Axion Solar Telescope
(CAST) has been searching for solar axions via their back conversion to X-ray
photons in a 9-T 10-m long magnet directed towards the Sun. We report on an
extended run with the IAXO (International Axion Observatory) pathfinder
detector, doubling the previous exposure time. The detector was operated with a
xenon-based gas mixture for part of the new run, providing technical insights
for future detector configurations in IAXO. No counts are detected in the 95\%
signal-encircling region during the new run, while one is expected. The new
data improve the axion-photon coupling limit to 5.7$\times
10^{-11}\,$GeV$^{-1}$ at 95\% C.L., the most restrictive experimental limit to
date.
