  Hexagonal YMnO$_{3}$ is a prototype antiferromagnet which exhibits
multiferroic behavior with the ferroelectric and magnetic transitions occurring
at different temperatures. We observe an out-of-plane canting of the Mn$^{3+}$
magnetic moments using resonant X-ray diffraction (RXD) in a single crystal of
this material. These canted moments result in the symmetry-forbidden (0,0,1)
magnetic Bragg reflection, which is observed at the Mn $L_{2,3}$ absorption
edges. We also observe an unexpected difference in the RXD spectral shapes at
different temperatures. Using ab initio calculations, we explore the
possibility that this behavior arises due to the interference between
scattering from the canted magnetic moments and parity-odd atomic multipoles on
the Mn$^{3+}$ ions.
