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 $y$.
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