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 $y \equiv m_e/m_p$, using the sum of the frequencies of cm-wave OH ``main'' lines. The technique is $\sim$ three orders of magnitude more sensitive than that of Chengalur & Kanekar (2003), which utilised only the four 18cm 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 18cm and 6cm lines can be combined with the redshift of an HCO$^+$ transition to measure any evolution in $\alpha$ and $y$. Both 18cm main lines and a number of HCO$^+$ lines have already been detected in absorption in four cosmologically distant systems; the detection of the main 6cm 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.