  The relation of period spacing ($\Delta P$) versus period ($P$) of dipole
prograde g modes is known to be useful to measure rotation rates in the g-mode
cavity of rapidly rotating $\gamma$ Dor and slowly pulsating B (SPB) stars. In
a rapidly rotating star, an inertial mode in the convective core can resonantly
couple with g modes propagative in the surrounding radiative region. The
resonant coupling causes a dip in the $P$-$\Delta P$ relation, distinct from
the modulations due to the chemical composition gradient. Such a resonance dip
in $\Delta P$ of prograde dipole g modes appears around a frequency
corresponding to a spin parameter $2f_{\rm rot}{\rm(cc)}/\nu_{\rm co-rot} \sim
8-11$ with $f_{\rm rot}$(cc) being the rotation frequency of the convective
core and $\nu_{\rm co-rot}$ the pulsation frequency in the co-rotating frame.
The spin parameter at the resonance depends somewhat on the extent of core
overshooting, central hydrogen abundance, and other stellar parameters. We can
fit the period at the observed dip with the prediction from prograde dipole g
modes of a main-sequence model, allowing the convective core to rotate
differentially from the surrounding g-mode cavity. We have performed such
fittings for 16 selected $\gamma$ Dor stars having well defined dips, and found
that the majority of $\gamma$ Dor stars we studied rotate nearly uniformly,
while convective cores tend to rotate slightly faster than the g-mode cavity in
less evolved stars.
