We report deep Green Bank Telescope spectroscopy in the redshifted H i 21cm and OH 18cm lines from the z = 0.765 absorption system towards PMN J0134 - 0931 .
A comparison between the “ satellite ” OH 18cm line redshifts , or between the redshifts of the H i 21cm and “ main ” OH 18cm lines , is sensitive to changes in different combinations of three fundamental constants , the fine structure constant \alpha , the proton-electron mass ratio \mu \equiv m _ { p } / m _ { e } and the proton g-factor g _ { p } .
We find that the satellite OH 18cm lines are not perfectly conjugate , with both different line shapes and stronger 1612 MHz absorption than 1720 MHz emission .
This implies that the satellite lines of this absorber are not suitable to probe fundamental constant evolution .
A comparison between the redshifts of the H i 21cm and OH 18cm lines , via a multi-Gaussian fit , yields the strong constraint \left [ \Delta F / F \right ] = [ -5.2 \pm 4.3 ] \times 10 ^ { -6 } , where F \equiv g _ { p } \left [ \mu \alpha ^ { 2 } \right ] ^ { 1.57 } and the error budget includes contributions from both statistical and systematic errors .
We thus find no evidence for a change in the constants between z = 0.765 and the present epoch .
Incorporating the constraint \left [ \Delta \mu / \mu \right ] < 3.6 \times 10 ^ { -7 } from another absorber at a similar redshift and assuming that fractional changes in g _ { p } are much smaller than those in \alpha , we obtain \left [ \Delta \alpha / \alpha \right ] = ( -1.7 \pm 1.4 ) \times 10 ^ { -6 } over a lookback time of 6.7 Gyrs .