  We report precise measurements of ground-state, $\lambda$-doublet microwave
transitions in the hydroxyl radical molecule (OH). Utilizing slow, cold
molecules produced by a Stark decelerator we have improved over the precision
of the previous best measurement by twenty-five-fold for the F' = 2 $\to$ F = 2
transition, yielding (1 667 358 996 $\pm$ 4) Hz, and by ten-fold for the F' = 1
$\to$ F = 1 transition, yielding (1 665 401 803 $\pm$ 12) Hz. Comparing these
laboratory frequencies to those from OH megamasers in interstellar space will
allow a sensitivity of 1 ppm for $\Delta\alpha/\alpha$ over $\sim$$10^{10}$
years.
