Im}}\tilde{\Pi}_{A}(t)\right),$$ where the tilde in ${\mbox{\rm Im}}\tilde{\Pi}$ indicates that the pion pole has been removed. Using the ansatz in eqs. (\[eq:Vresdomqcd\]) and (\[eq:Aresdomqcd\]) and the two Weinberg sum rules in eq. (\[eq:tsc\]) one finds $$-4L_{10}=f_{V}^2-f_{A}^2=f_{\pi}^2\left(\frac{1}{M_{V}^2}+\frac{1}
{M_{A}^2}\right),$$ which provides a good estimate [@EGLPR89] of the $L_{10}$ coupling constant.

iv\) One of the parameters which characterize possible deviations from the Standard Model predictions in the electroweak sector is the so called $S$–parameter \[[See the lectures of Treille [@TrelesH] and Chivukula [@ChilesH] in this school.]{}\]. In the unitary gauge, $S$ measures the strength of an anomalous $W^{(3)}_{\mu\nu}B^{\mu\nu}$ coupling. This is the $SU(2)_{L}\times SU(2)_{R}$ coupling analogous to the term proportional to $L_{10}$ in QCD. It has been argued that if the underlying theory of electroweak breaking is a vector–like gauge theory of the QCD–type, the sign of the $L_{10}$–like coupling must be negative, precisely as observed in QCD. In the electroweak sector, this fact is infirmed by experimental observation and constitutes at present a serious phenomenological obstacle to technicolour–like formulations of electroweak symmetry breaking. The relation of the $L_{10}$ coupling to the relative size of local order parameters versus $f_{\pi}^2$ in the large–$N_c$ limit of QCD–like theories has been recently discussed in ref. [@KdeR97].

