for the observed period spacing series of KIC 9751996, assuming l = 1, as a function of the asymptotic period spacing ΔΠ_l and the rotation rate f_rot. The white crosses indicate the best-fit solutions. Top: the χ²-distribution that we find by only analysing the detected prograde series. Bottom: the χ²-distribution obtained by fitting the prograde, zonal, and retrograde spacing series simultaneously.