Analysis of KIC 7023917 -- spotted low-mass ratio eclipsing binary with $δ$ Scuti pulsations

Times of minima of eclipsing binary KIC 7023917 show quasiperiodic anti-symmetric deviations from the calculated one with an amplitude of up to 10 minutes and a period of 200 - 300 days. These changes correlate with the observed variations of the light-curve maxima (amplitude and phase separation). We used photometric data obtained by Kepler and TESS missions to analyse the times of minima and determine system parameters. The phases and amplitudes of the maxima were measured to study the O'Connell effect. As an additional source of information, we performed ground-based multi-colour photometric observation and determined the radial velocities of the system from our spectroscopic measurements. We could explain long-term variations of the light-curve shape and times of the eclipses using the cold star spot located on the secondary component and the modification of its size. Based on our modelling, the system consists of a primary main-sequence star of spectral type A7 and an evolved, oversized secondary component with a mass ratio of only 0.1 due to past mass transfer. Calculation of absolute parameters gives us the mass of the primary component about 1.8 M$_\odot$ and 0.2 M$_\odot$ for the secondary one, and radii of 2.2 R$_\odot$ of the primary star and 0.9 R$_\odot$ of secondary one, respectively. The studied low-mass ratio eclipsing binary is probably a progenitor of the variable star of EL CVn type. A multiple-period photometric variability was disclosed in the TESS data ranging from half to two hours due to $δ$ Scuti-type pulsations of the primary component.