  Calculations are presented for four relatively strong first-forbidden $\beta$
decays in the region A=11-16 in order to study the very large
mesonic-exchange-current enhancement of the rank-zero components. The $\mu^-$
capture on $^{16}$O is considered on the same footing. The wave functions
utilized include up to 4$\hbar\omega$ excitations. Two-body exchange-current
matrix elements are calculated as well as one-body impulse approximation matrix
elements. The resultant enhancement factor that multiplies the
impulse-approximation axial-charge matrix element is thereby determined by
comparison to experiment to be $\epsilon_{exp}$ = 1.61 $\pm$ 0.03 from three
$\beta^-$ decays and $\mu^-$ capture on $^{16}$O, which is in excellent
agreement with meson-exchange calculations in the soft-pion approximation.
