I will describe a new technique to estimate variations in fundamental constants using multiple radio OH lines. This has the advantage that all lines arise in the same species, allowing a relatively clean comparison between the measured redshifts. Further, the technique allows a simultaneous measurement of changes in alpha, g_p and m_e/m_p, using OH lines from a single object. I will also present the first detection of the 18cm OH satellite lines at a cosmological distance, from the z~0.25 source PKS 1413+135. The 1720 MHz and 1620 MHz profiles are conjugate, in that the sum of their optical depths is consistent with noise; this implies that the lines arise in precisely the same gas. The redshifts of the line peaks are found to agree with each other, within the error bars; this is used to place constraints on changes in a combination of alpha, g_p and m_e/m_p over the redshift range 0 < z < 0.25.