  High magnetic fields were used to deform spherical nanocapsules,
self-assembled from bola-amphiphilic sexithiophene molecules. At low fields the
deformation -- measured through linear birefringence -- scales quadratically
with the capsule radius and with the magnetic field strength. These data
confirm a long standing theoretical prediction (W. Helfrich, Phys. Lett. {\bf
43A}, 409 (1973)), and permits the determination of the bending rigidity of the
capsules as (2.6$\pm$0.8)$\times 10^{-21}$ J. At high fields, an enhanced
rigidity is found which cannot be explained within the Helfrich model. We
propose a complete form of the free energy functional that accounts for this
behaviour, and allows discussion of the formation and stability of nanocapsules
in solution.
