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We used high-quality Kepler photometry and spectroscopic data to investigatethe Kepler binary candidate KIC 5988140. Using the spectrum synthesis method,we derived the fundamental parameters Teff, log g, [M/H], and v.sini and theabundances. Frequency analyses of both the photometric and the spectroscopicdata were performed, revealing the same two dominant frequencies (F_1=0.688 andF_2=0.344 c/d). We also detected in the photometry the signal of nine more,significant frequencies located in the typical range of Delta Scuti pulsation.The light and radial velocity curves follow a similar, stable double-wavepattern which are not exactly in anti-phase but show a relative phase shift ofabout 0.1 period between the moment of minimum velocity and that of maximumlight. We considered three different scenarios: binarity, co-existence of bothGamma Doradus and Delta Scuti pulsations and rotation of the stellar surfacewith an axisymmetric intensity distribution. However, none of these scenariosis capable of explaining all of the characteristics of the observed variations.We confirm the occurrence of various independent Delta Scuti-type pressuremodes in the Kepler light curve. With respect to the low-frequency content,however, we argue that the physical cause of the remaining light and radialvelocity variations of this late A-type star remains unexplained by any of thepresently considered scenarios.

Low-frequency variations of unknown origin in the Kepler Delta Scuti star KIC 5988140 = HD 188774