  The CERN Resonant WISP Search (CROWS) probes the existence of Weakly
Interacting Sub-eV Particles (WISPs) like axions or hidden sector photons. It
is based on the principle of an optical light shining through the wall
experiment, adapted to microwaves. Critical aspects of the experiment are
electromagnetic shielding, design and operation of low loss cavity resonators
and the detection of weak sinusoidal microwave signals. Lower bounds were set
on the coupling constant $g = 4.5 \cdot 10^{-8} $GeV$^{-1}$ for axion like
particles with a mass of $m_a = 7.2 \mu$eV. For hidden sector photons, lower
bounds were set for the coupling constant $\chi = 4.1 \cdot 10^{-9}$ at a mass
of $m_{\gamma'} = 10.8 \mu$eV. For the latter we were probing a previously
unexplored region in the parameter space.
