2004MNRAS.350L..17K

We describe a new technique to measure variations in the fundamental parameters α and y≡m_e/m_p, using the sum of the frequencies of cm-wave OH `main' lines. The technique is ∼ three orders of magnitude more sensitive than that of Chengalur & Kanekar, which utilized only the four 18-cm OH lines. The increase in sensitivity stems from the use of OH `main' lines arising from different rotational states, instead of the frequency difference between lines from the same state. We also show that redshifts of the main OH 18- and 6-cm lines can be combined with the redshift of an HCO⁺ transition to measure any evolution in α and y. Both 18-cm main lines and a number of HCO⁺ lines have already been detected in absorption in four cosmologically distant systems; the detection of the main 6-cm OH line in any of these systems would thus be sufficient to simultaneously constrain changes in α and y between the absorption redshift and today.