  Deformations that conserve the parallelism and the distances --between
layers, in smectic phases; between columns, in columnar phases-- are
commonplace in liquid crystals. The resulting deformed textures have the same
mass density as in the ground state (an expected property in a liquid) and are
at the same time isogonic and isometric, which imposes specific geometric
features. The corresponding order parameter singularities extend over rather
large, macroscopic, distances, e.g., cofocal conics in smectics. This
well-known picture is modified when, superimposed to the 1D or 2D
periodicities, the structure is helical. Isogony is no longer the rule, but
isometry (and mass density) can be preserved. This paper discusses the case of
a medium whose structure is made of 1D modulated layers (a lamello-columnar
phase), assuming that the modulations rotate helically from one layer to the
next. The price to pay is that any isometric texture is necessarily frustrated;
it consists of layers folded into a set of parallel helicoids, in the manner of
a screw dislocation (of macroscopic Burgers vector), the modulations being
along the helices, i.e. double-twisted. The singularity set is made of two
helical disclination lines. We complete this geometric analysis by a crude
calculation of the energy of a helical ribbon. It is suggested that the helical
ribbons observed in the B7 phase of banana-like molecules are such isometric
textures. As a side result, let us mention that the description of
double-twist, traditionally made in terms of a partition of the director field
into nested cylinders, could more than often be profitably tested against a
partition into nested helicoids.
