  We used high-quality Kepler photometry and spectroscopic data to investigate
the Kepler binary candidate KIC 5988140. Using the spectrum synthesis method,
we derived the fundamental parameters Teff, log g, [M/H], and v.sini and the
abundances. Frequency analyses of both the photometric and the spectroscopic
data were performed, revealing the same two dominant frequencies (F_1=0.688 and
F_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-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. We considered three different scenarios: binarity, co-existence of both
Gamma Doradus and Delta Scuti pulsations and rotation of the stellar surface
with an axisymmetric intensity distribution. However, none of these scenarios
is capable of explaining all of the characteristics of the observed variations.
We confirm the occurrence of various independent Delta Scuti-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.
