  We report deep Green Bank Telescope spectroscopy in the redshifted HI 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 HI 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 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 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 $[\Delta \mu/\mu ] <
3.6 \times 10^{-7}$ from another absorber at a similar redshift and assuming
that fractional changes in $g_p$ are much smaller than those in $\alpha$, we
obtain $[\Delta \alpha/\alpha ] = (-1.7 \pm 1.4) \times 10^{-6}$ over a
lookback time of 6.7 Gyrs.
