  We present a new study of SU(3)-breaking corrections in hyperon vector form
factors relevant for the extraction of Vus. A lattice quenched simulation has
been performed, showing that it is possible to reach the required precision to
extract SU(3)-breaking corrections in the regime of simulated masses. In order
to perform the chiral extrapolation we calculated the chiral corrections to the
vector form factor in HBChPT. Besides the one-loop O(p^2) contribution, we
included also the subleading O(p^3) and O(1/M_B) corrections that, due to the
Ademollo-Gatto theorem, are free from the contamination of unknown low energy
constants. The results complete and correct previous calculations, and show
that subleading corrections cannot be neglected. We also studied decuplet
contributions within HBChPT and show that, in this case, the chiral expansion
breaks down, rising doubts on the consistency of the theory.
