The use of OH 'main' lines to constrain the variation of fundamental constants

We describe a new technique to measure variations in the fundamental parameters alpha and yequivalent tom(e)/m(p), using the sum of the frequencies of cm-wave OH 'main' lines. The technique is similar to 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 alpha 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 alpha and y between the absorption redshift and today.