Combination frequencies in high-amplitude δ Scuti stars

Short-cadence observations of delta Scuti stars in the Kepler field are used to investigate the physical nature of high-amplitude d Scuti stars (HADS). Although it is often mentioned that HADS are transition objects between classical Cepheids and delta Scuti stars, neither ground-based or space-based observations support this view. It is found that HADS occur randomly within the instability strip. The possibility that HADS may be defined by the presence of combination frequencies is discussed. There is a weak tendency for the number of combination frequencies to increase with increasing amplitude of the parent frequencies. However, even stars with very low amplitudes may have detectable combination frequencies. Very few parent modes have a period ratio appropriate to first-overtone and fundamental radial modes. It appears that a high amplitude, in itself, is not useful as a distinguishing feature of d Scuti stars.