  Fractional Josephson vortices carry a magnetic flux Phi, which is a fraction
of the magnetic flux quantum Phi_0 ~ 2.07x10^{-15} Wb. Their properties are
very different from the properties of the usual integer fluxons. In particular,
fractional vortices are pinned and have an oscillation eigenfrequency which is
expected to be within the Josephson plasma gap. Using microwave spectroscopy,
we investigate the dependence of the eigenfrequency of a fractional Josephson
vortex on its magnetic flux $\Phi$ and on the bias current. The experimental
results are in good agreement with the theoretical predictions.
