  We have searched for solar axions or other pseudoscalar particles that couple
to two photons by using the CERN Axion Solar Telescope (CAST) setup. Whereas we
previously have reported results from CAST with evacuated magnet bores (Phase
I), setting limits on lower mass axions, here we report results from CAST where
the magnet bores were filled with \hefour gas (Phase II) of variable pressure.
The introduction of gas generated a refractive photon mass $m_\gamma$, thereby
achieving the maximum possible conversion rate for those axion masses \ma that
match $m_\gamma$. With 160 different pressure settings we have scanned \ma up
to about 0.4 eV, taking approximately 2 h of data for each setting. From the
absence of excess X-rays when the magnet was pointing to the Sun, we set a
typical upper limit on the axion-photon coupling of $\gag\lesssim 2.17\times
10^{-10} {\rm GeV}^{-1}$ at 95% CL for $\ma \lesssim 0.4$ eV, the exact result
depending on the pressure setting. The excluded parameter range covers
realistic axion models with a Peccei-Quinn scale in the neighborhood of $f_{\rm
a}\sim10^{7}$ GeV. Currently in the second part of CAST Phase II, we are
searching for axions with masses up to about 1.2 eV using \hethree as a buffer
gas.
