  We report Westerbork Synthesis Radio Telescope and Arecibo Telescope
observations of the redshifted satellite OH-18cm lines at $z \sim 0.247$
towards PKS1413+135. The "conjugate" nature of these lines, with one line in
emission and the other in absorption, but with the same shape, implies that the
lines arise in the same gas. The satellite OH-18cm line frequencies also have
different dependences on the fine structure constant $\alpha$, the
proton-electron mass ratio $\mu = m_p/m_e$, and the proton gyromagnetic ratio
$g_p$. Comparisons between the satellite line redshifts in conjugate systems
can hence be used to probe changes in $\alpha$, $\mu$, and $g_p$, with few
systematic effects. The technique yields the expected null result when applied
to Cen.A, a nearby conjugate satellite system. For the $z \sim 0.247$ system
towards PKS1413+135, we find, on combining results from the two telescopes,
that $[\Delta G/G] = (-1.18 \pm 0.46) \times 10^{-5}$ (weighted mean), where $G
= g_p [\mu \alpha^2]^{1.85}$; this is tentative evidence (with $2.6 \sigma$
significance, or at 99.1% confidence) for a smaller value of $\alpha$, $\mu$,
and/or $g_p$ at z~0.247, i.e. at a lookback time of ~2.9 Gyrs. If we assume
that the dominant change is in $\alpha$, this implies $[\Delta \alpha /\alpha ]
= (-3.1 \pm 1.2) \times 10^{-6}$. We find no evidence that the observed offset
might be produced by systematic effects, either due to observational or
analysis procedures, or local conditions in the molecular cloud.
