Enhanced effect of temporal variation of the fine structure constant in   diatomic molecules
  We show that the relative effect of variation of the fine structure constant in microwave transitions between very close and narrow rotational-hyperfine levels may be enhanced 2-3 orders of magnitude in diatomic molecules like LaS, LaO, LuS, YbF, etc. The enhancement is a result of cancellation between the hyperfine and rotational intervals. 
V.V. Flambaum