$\Lambda$-doublet spectra of diatomic radicals and their dependence on fundamental constants

$\Lambda$-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 $10^3$.