Do the Fundamental Constants change with Time ?

Radio spectroscopy in the multiple redshifted OH 18cm lines provides a powerful probe of changes in the fine structure constant α, the proton g-factor gp and the proton-electron mass ratio mp/me. Under certain astrophysical conditions, a maser mechanism causes the satellite OH 18cm lines to be ``conjugate'', with one line in absorption, the other in emission and the sum of the optical depths consistent with noise. This implies that the lines arise in precisely the same gas, making them ideal transitions through which to study changes in α, μ and gp,with few systematic effects. In this talk, I will present results from deep WSRT and Arecibo studies of a redshifted conjugate satellite OH system, at z  ∼ 0.25 towards PKS 1413+135. I will also describe results from an alternative radio technique, comparing redshifts of inversion and rotationl transitions, that has yielded the best sensitivity today to changes in the proton-electron mass ratio.