  We investigate the instability of a tubular fluid membranes made of a water
soluble surfactant. The tubules are obtained at high salinities. The
instability is due to the introduction within the vesicle multilayer of an
alkane. We measure the wavelength of this instability versus the unperturbed
radius of the tubules. To interpretthis dependance we use a model that includes
not only the surface tension in the elastic energy but the spontaneous
curvature as well. The spontaneous curvature is induced by the presenceof the
oil in the bilayer of the membrane and give a selection of a non-zero
wavelength.
