We present the complete calculation of the SU(3)-breaking corrections to the
hyperon vector form factors up to O(p^4) in the Heavy Baryon Chiral
Perturbation Theory. Because of the Ademollo-Gatto theorem, at this order the
results do not depend on unknown low energy constants and allow to test the
convergence of the chiral expansion. We complete and correct previous
calculations and find that O(p^3) and O(1/M_0) corrections are important. We
also study the inclusion of the decuplet degrees of freedom, showing that in
this case the perturbative expansion is jeopardized. These results raise doubts
on the reliability of the chiral expansion for hyperons.Comment: 20 pages, 4 figures, v2: published versio