$\chi^2$-distribution that we find by only analysing the detected prograde series. *Bottom:* the $\chi^2$-distribution obtained by fitting the prograde, zonal and retrograde spacing series simultaneously.](KIC9751996_chi2comp.pdf){width="88mm"}

![\[fig:kic9751996\_fit\] The observed period spacing patterns (black dots and grey triangles) for the retrograde (top), zonal (middle) and prograde (bottom) modes of KIC9751996. The black parts of the input patterns were used to determine $f_{rot}$ and $\Delta\Pi_{l}$, while the grey sections were excluded. The white squares indicate the modes of the optimal model in the grid when all three series are fitted simultaneously.](KIC9751996_fit.pdf){width="88mm"}

Next we also analysed the period spacing patterns of KIC12066947, a fast rotating star for which both a prograde and a retrograde period spacing series were detected. While we were able to fit the pattern of prograde modes to derive a rotation rate $f_{rot}$, the observed retrograde series presented us with a challenge. We found these to correspond with Rossby modes rather than “classical” gravity or gravito-inertial modes. Rossby modes can only occur in rotating stars and originate from the interaction between the stellar rotation and toroidal modes [e.g. @Papaloizou1978; @Townsend2003c]. Our identification of the retrograde modes as Rossby modes is illustrated in the top panel of Fig.\[fig:kic12066947\_retro\], where we show the 