  Recently many authors have performed shell-model calculations of nuclear
matrix elements determining the rates of the ordinary muon capture in light
nuclei. These calculations have employed well-tested effective interactions in
large scale shell-model studies. For one of the nuclei of interest, namely
$^{28}$Si, there exists recent experimental data which can be used to deduce
the value of the ratio $C_{P}/C_{A}$ by using the calculated matrix elements.
Surprisingly enough, all the abovementioned shell-model results suggest a very
small value ($\simeq 0$) for $C_{P}/C_{A}$, quite far from the PCAC prediction
and recent data on muon capture in hydrogen. We show that this rather
disturbing anomaly is solved by employing effective transition operators. This
finding is also very important in studies of the scalar coupling of the weak
charged current of leptons and hadrons.
