  Using dispersion theory the low-energy electromagnetic form factors for the
transition of a Sigma to a Lambda hyperon are related to the pion vector form
factor. The additionally required input, i.e. the two-pion--Sigma--Lambda
amplitudes are determined from relativistic next-to-leading-order (NLO) baryon
chiral perturbation theory including the baryons from the octet and optionally
from the decuplet. Pion rescattering is again taken into account by dispersion
theory. It turns out that the inclusion of decuplet baryons is not an option
but a necessity to obtain reasonable results. The electric transition form
factor remains very small in the whole low-energy region. The magnetic
transition form factor depends strongly on one not very well determined
low-energy constant of the NLO Lagrangian. One obtains reasonable predictive
power if this low-energy constant is determined from a measurement of the
magnetic transition radius. Such a measurement can be performed at the future
Facility for Antiproton and Ion Research (FAIR).
