Résumé
We observe the defects separating nematic domains of two easy orientations, above a bistable orienting SiO evaporated glass plate. The defects are coreless walls. Their width depends on the bulk texture imposed by the nematic cell geometry. For thin enough samples, we have found situations where the wall thickness diverges, i.e., where the texture can force a surface orientation exactly along the surface barrier orientation. We model this new kind of surface transition. The barrier energy of the bistable anchoring is found small, with an extrapolation length $\approx 1~\mu$m. We observe also hysteresis effects which can be explained by a more or less permanent first molecular layer adsorbed on the surface. The corresponding non-elastic contribution to the anchoring is one order of magnitude larger than the elastic one.