We report Westerbork Synthesis Radio Telescope and Arecibo Telescopeobservations of the redshifted satellite OH-18cm lines at $z \sim 0.247$towards PKS1413+135. The "conjugate" nature of these lines, with one line inemission and the other in absorption, but with the same shape, implies that thelines arise in the same gas. The satellite OH-18cm line frequencies also havedifferent dependences on the fine structure constant $\alpha$, theproton-electron mass ratio $\mu = m_p/m_e$, and the proton gyromagnetic ratio$g_p$. Comparisons between the satellite line redshifts in conjugate systemscan hence be used to probe changes in $\alpha$, $\mu$, and $g_p$, with fewsystematic effects. The technique yields the expected null result when appliedto Cen.A, a nearby conjugate satellite system. For the $z \sim 0.247$ systemtowards 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 assumethat 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 offsetmight be produced by systematic effects, either due to observational oranalysis procedures, or local conditions in the molecular cloud.
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