2020A&A...638A..57S

Aims. KIC 8975515 is a double-lined spectroscopic binary system with hybrid nature. Regular monitoring with the echelle spectrographHERMES show that both components have matching atmospheric properties (T_eff∼7400K), and that one of them is a fast rotator (vsini=162 versus 32km/s). Our aim is to study the Kepler light curve in order to determine the frequencies of the pulsations, to search for regular spacing patterns in the Fourier spectrum, if any, and to discuss their origin in the context of binarity and fast rotation.
Methods. In this paper, we study the properties of the stellar pulsations based on a careful analysis of the Fourier spectrum. This was done by performing repeated frequency-search analyses with successive prewhitenings of all the significant frequencies detected in the spectrum. Moreover, we searched for regular period spacing among the g modes as well as frequency splitting among the g and p modes in frequency.
Results. In the low-frequency regime, one series of prograde g modes and four series of retrograde r modes, were detected. The r modes are well-distributed with respect to the harmonics of the rotational frequency of the fast-rotating star f_rot=1.647d^–1. The dominant g mode is f₂=2.37d^–1. The strongest p mode is f₁=13.97d^–1. We identified two multiplets of regularly split p modes with the mean values of 0.42d^–1 and 1.65d^–1. In addition, we also identified a few incidences of mode coupling (between f₁ and a g or r mode).
Conclusions. We detected five series of retrograde r and prograde g modes as well as two multiplets of p modes with rotational frequency splitttings related to both components of KIC 8975515. We identified the fast-rotating component as a hybrid pulsator with r modes, and the slowly-rotating component as a δ Sct pulsator.