Constraining the variation of fundamental constants using 18cm OH lines

We describe a new technique to estimate variations in the fundamental constants using 18cm OH absorption lines. This has the advantage that all lines arise in the same species, allowing a clean comparison between the measured redshifts. In conjunction with one additional transition (for example, an HCO$^+$ line), it is possible to simultaneously measure changes in $\alpha$, $g_p$ and $y \equiv m_e/m_p$. At present, only the 1665 MHz and 1667 MHz lines have been detected at cosmological distances; we use these line redshifts in conjunction with those of HI 21cm and mm-wave molecular absorption in a gravitational lens at $z\sim 0.68$ to constrain changes in the above three parameters over the redshift range $0 < z \lesssim 0.68$. While the constraints are relatively weak ($\lesssim$ 1 part in $10^3$), this is the first simultaneous constraint on the variation of all three parameters. We also demonstrate that either one (or more) of $\alpha$, $g_p$ and $y$ must vary with cosmological time or there must be systematic velocity offsets between the OH, HCO$^+$ and HI absorbing clouds.