  PN G135.9+55.9 is a compact, high excitation nebula that has been identified
recently as the most oxygen-poor halo planetary nebula. Given its very peculiar
characteristics and potential implications in the realms of stellar and
Galactic evolution, additional data are needed to firmly establish its true
nature and evolutionary history. Here we present the first long-slit, high
spectral resolution observations of this object in the lines of H$\alpha$ and
He II 4686. The position-velocity data are shown to be compatible with the
interpretation of PN G135.9+55.9 being a halo planetary nebula. In both
emission lines, we find the same two velocity components that characterize the
kinematics as that of an expanding elliptical envelope. The kinematics is
consistent with a prolate ellipsoidal model with axis ratio about 2:1, a
radially decreasing emissivity distribution, a velocity distribution that is
radial, and an expansion velocity of 30 km/s for the bulk of the material. To
fit the observed line profiles, this model requires an asymmetric matter
distribution, with the blue-shifted emission considerably stronger than the
red-shifted emission. We find that the widths of the two velocity components
are substantially wider than those expected due to thermal motions, but
kinematic structure in the projected area covered by the slit appears to be
sufficient to explain the line widths. The present data also rule out the
possible presence of an accretion disk in the system that could have been
responsible for a fraction of the H$\alpha$ flux, further supporting the
planetary nebula nature of PN G135.9+55.9.
