  We present an experimental study of the directional-solidification patterns
of a nematic - smectic B front. The chosen system is C_4H_9-(C_6H_{10})_2CN (in
short, CCH4) in 12 \mu m-thick samples, and in the planar configuration
(director parallel to the plane of the sample). The nematic - smectic B
interface presents a facet in one direction -- the direction parallel to the
smectic layers -- and is otherwise rough, and devoid of forbidden directions.
We measure the Mullins-Sekerka instability threshold and establish the
morphology diagram of the system as a function of the solidification rate V and
the angle theta_{0} between the facet and the isotherms. We focus on the
phenomena occurring immediately above the instability threshold when theta_{0}
is neither very small nor close to 90^{o}. Under these conditions we observe
drifting shallow cells and a new type of solitary wave, called "faceton", which
consists essentially of an isolated macroscopic facet traveling laterally at
such a velocity that its growth rate with respect to the liquid is small.
Facetons may propagate either in a stationary, or an oscillatory way. The
detailed study of their dynamics casts light on the microscopic growth
mechanisms of the facets in this system.
