  It is usual to think of Focal Conic Domains (FCD) as perfect geometric
constructions in which the layers are folded into Dupin cyclides, about an
ellipse and a hyperbola that are conjugate. This ideal picture is often far
from reality. We have investigated in detail the FCDs in several materials that
have a transition from a smectic A (SmA) to a nematic phase (N). The ellipse
and the hyperbola are seldom perfect, and the FCD textures also suffer large
transformations (in shape or/and in nature) when approaching the transition to
the nematic phase, or appear imperfect on cooling from the nematic phase. We
interpret these imperfections as due to the interaction of FCDs with
dislocations. We analyze theoretically the general principles subtending the
interaction mechanisms between FCDs and finite Burgers vector dislocations,
namely the formation of \textit{kinks} on disclinations, to which dislocations
are attached, and we present models relating to some experimental results.
Whereas the principles of the interactions are very general, their realizations
can differ widely in function of the boundary conditions.
