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.
