  $\Lambda$-doublet spectra of light diatomic radicals have high sensitivity to
the possible variations of the fine structure constant alpha and
electron-to-proton mass ratio beta. For molecules OH and CH sensitivity is
further enhanced because of the J-dependent decoupling of the electron spin
from the molecular axis, where J is total angular momentum of the molecule.
When $\Lambda$-splitting has different signs in two limiting coupling cases a
and b, decoupling of the spin leads to the change of sign of the splitting and
to the growth of the dimensionless sensitivity coefficients. For example,
sensitivity coefficients for the $\Lambda$-doublet lines J=9/2 of the
$\Pi_{1/2}$ state of OH molecule are on the order of $10^3$.
