  We have detected the two 18cm OH satellite lines from the $z \sim 0.247$
source PKS1413+135, the 1720 MHz line in emission and the 1612 MHz line in
absorption. The 1720 MHz luminosity is $L_{\rm OH} \sim 354 L_\odot$, more than
an order of magnitude larger than that of any other known 1720 MHz maser. The
profiles of the two satellite lines are conjugate, implying that they arise in
the same gas. This allows us to test for any changes in the values of
fundamental constants, without being affected by systematic uncertainties
arising from relative motions between the gas clouds in which the different
lines arise. Our data constrain changes in $G \equiv g_p [\alpha^2/y]^{1.849}$,
where $ y \equiv m_e/m_p$; we find $\Delta G/G = 2.2 \pm 3.8 \times 10^{-5}$,
consistent with no changes in $\alpha$, $g_p$ and $y$.
