Stellar seismology of δ Scuti stars offers the best prospect for determining the interior properties of normal non-magnetic A-F stars on or near the main sequence. The discovery of rich eigenfrequency spectra in the light variations of several δ Scuti stars (e.g., θ
2 Tau: Breger et al. 1989; GX Peg: Michel et al. 1990) should make this possible for low-degree modes (l ≤ 4). However, due to cancellation effects, modes of higher l cannot be studied through photometry or radial velocity measurements.
The detection of rapid line-profile variations among rapidly rotating δ Scuti stars (e.g., Walker et al. 1987; Kennelly et al. 1992) has extended our sensitivity to much higher modes. These variations are consistent with moderate- to high-degree modes, where |m| ~ l. (In the simplest case, they could be pure sectorial modes with |m| = l and pulsation amplitudes strongly confined to the stellar equator.) To use these data effectively, we need an objective way to identify the modes and their time evolution.