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We report deep Green Bank Telescope spectroscopy in the redshifted HI 21cmand OH 18cm lines from the $z = 0.765$ absorption system towards PMNJ0134-0931. A comparison between the "satellite" OH 18cm line redshifts, orbetween the redshifts of the HI 21cm and "main" OH 18cm lines, is sensitive tochanges in different combinations of three fundamental constants, the finestructure constant $\alpha$, the proton-electron mass ratio $\mu \equivm_p/m_e$ and the proton g-factor $g_p$. We find that the satellite OH 18cmlines are not perfectly conjugate, with both different line shapes and stronger1612 MHz absorption than 1720 MHz emission. This implies that the satellitelines of this absorber are not suitable to probe fundamental constantevolution. A comparison between the redshifts of the HI 21cm and OH 18cm lines,via a multi-Gaussian fit, yields the strong constraint $[\Delta F/F] = [-5.2\pm 4.3] \times 10^{-6}$, where $F \equiv g_p [\mu \alpha^2]^{1.57}$ and theerror budget includes contributions from both statistical and systematicerrors. We thus find no evidence for a change in the constants between $z =0.765$ and the present epoch. Incorporating the constraint $[\Delta \mu/\mu ] <3.6 \times 10^{-7}$ from another absorber at a similar redshift and assumingthat fractional changes in $g_p$ are much smaller than those in $\alpha$, weobtain $[\Delta \alpha/\alpha ] = (-1.7 \pm 1.4) \times 10^{-6}$ over alookback time of 6.7 Gyrs.

Constraining fundamental constant evolution with HI and OH lines