Low-frequency variations of unknown origin in the Kepler δ Scuti star KIC 5988140=HD188774

Context. The NASA exoplanet search mission Kepler is currently providing a wealth of light curves of ultra-high quality from space. Aims. We used high-quality Kepler photometry and spectroscopic data to investigate the Kepler target and binary candidate KIC5988140. We aim to interpret the observed variations of KIC5988140 considering three possible scenarios: binarity, co-existence of delta Sct- and gamma Dor-type oscillations, and rotational modulation caused by an asymmetric surface intensity distribution. Methods. We used the spectrum synthesis method to derive the fundamental parameters T-eff, log g, [M/H], and v sin i from the newly obtained high-resolution, high S/N spectra. Frequency analyses of both the photometric and the spectroscopic data were performed. Results. The star has a spectral type of A7.5 IV-III and a metallicity slightly lower than that of the Sun. Both Fourier analyses reveal the same two dominant frequencies F-1 = 2F(2) = 0.688 and F-2 = 0.344 d(-1). We also detected in the photometry the signal of nine more, significant frequencies located in the typical range of delta Sct pulsation. The light and radial velocity curves follow a similar, stable double-wave pattern which are not exactly in anti-phase but show a relative phase shift of about 0.1 period between the moment of minimum velocity and that of maximum light. Conclusions. Such findings are incompatible with the star being a binary system. We next show that, for all possible (limit) configurations of a spotted surface, the predicted light-to-velocity amplitude ratio is almost two orders larger than the observed value, which pleads against rotational modulation. The same argument also invalidates the explanation in terms of pulsations of type gamma Dor (i.e. hybrid pulsations). We confirm the occurrence of various independent d Sct- type pressure modes in the Kepler light curve. With respect to the low-frequency content, however, we argue that the physical cause of the remaining light and radial velocity variations of this late A-type star remains unexplained by any of the presently considered scenarios.