Asteroseismology of HADS stars: V974 Oph, a radial pulsator flavoured by nonradial components

The analysis of a dense time-series on V974 Oph disclosed the rich pulsational content (at least five independent terms) of this high-amplitude (0.60 mag in B–light) δ Sct star. A mode with a frequency very close to the main one (probably the fundamental radial mode) has been detected: such a doublet is not a common feature in stars of the same class. Also another term can be considered a radial one, but the high ratio (0.786) raises some problems that can be solved only by admitting very low metallicity. It is quite evident that some undetectable terms are again hidden in the noise, as the least-squares fit leaves a rms residual much higher than the observational noise. All that considered, nonradial modes seem to play a key rôle in the light variability of V974 Oph. Revealing an unsuspected asteroseismic interest, V974 Oph provides a link between low- and high-amplitude δ Sct stars