Are second-class currents present in hyperon beta decay?

Within the framework of Cabibbo $V-A$ theory bounds are obtained on the pseudotensor form factor which is induced by SU(3) violations. Such an estimate is necessary to distinguish second-class effects from effects of SU(3) violations. It is argued that calculations based on an analogy with the dispersion-theory evaluation of the nucleon anomalous magnetic moment are suspect since the vector and axial-vector currents have very different structures. A different dispersion-theory calculation involving only the matrix elements of the axial-vector currents is presented, which predicts the $\frac{F}{D}$ ratio of the pion-nucleon octet coupling to be $\pm {\left(\frac{5}{9}\right)}^{\frac{1}{2}}$. It is concluded that, if the controversial large experimental value of this term is confirmed, this would then require the presence of a second-class current.