  Asteroseismic inference from pressure modes (p modes) and buoyancy, or
gravity, modes (g modes) is ubiquitous for stars across the
Hertzsprung--Russell diagram. Until now, however, discussion of r modes (global
Rossby waves) has been rare. Here we derive the expected frequency ranges of r
modes in the observational frame by considering the visibility of these modes.
We find that the frequencies of r modes of azimuthal order $m$ appear as groups
at slightly lower frequency than $m$ times the rotation frequency. Comparing
the visibility curves for r modes with Fourier amplitude spectra of Kepler
light curves of upper main-sequence B, A and F stars, we find that r modes are
present in many $\gamma$ Dor stars (as first discovered by Van Reeth et al.
2016), spotted stars, and so-called Heartbeat stars, which are highly eccentric
binary stars. We also find a signature of r modes in a frequently bursting Be
star observed by Kepler. In the amplitude spectra of moderately to rapidly
rotating $\gamma$ Dor stars, r-mode frequency groups appear at lower frequency
than prograde g-mode frequency groups, while in the amplitude spectra of
spotted early A to B stars, groups of symmetric (with respect to the equator)
r-mode frequencies appear just below the frequency of a structured peak that we
suggest represents an approximate stellar rotation rate. In many Heartbeat
stars, a group of frequencies can be fitted with symmetric $m=1$ r modes, which
can be used to obtain rotation frequencies of these stars.
