  I searched for the ground state 6.8 and 9.2 GHz hyperfine transitions of
rubidium and cesium toward M- and L-dwarfs that show Rb and Cs optical
resonance lines. The optical lines can pump the hyperfine transitions,
potentially forming masers. These spin-flip transitions of Rb and Cs are the
principal transitions used in atomic clocks (the $^{133}$Cs hyperfine
transition defines the second). If they are detected in stellar atmospheres,
these transitions would provide exceptionally precise clocks that can be used
as accelerometers, as exoplanet detectors, as probes of the predictions of
general relativity, as probes of light propagation effects, and as a means to
do fundamental physics with telescopes. Observations of 21 M- and L-dwarfs,
however, show no evidence for Rb or Cs maser action, and a previous survey of
giant stars made no Rb maser detections.
