  Among 19 red-giant stars belonging to eclipsing binary systems that have been
identified in Kepler data, 15 display solar-like oscillations. We study whether
the absence of mode detection in the remaining 4 is an observational bias or
possibly evidence of mode damping that originates from tidal interactions. A
careful analysis of the corresponding Kepler light curves shows that modes with
amplitudes that are usually observed in red giants would have been detected if
they were present. We observe that mode depletion is strongly associated with
short-period systems, in which stellar radii account for 16-24 % of the
semi-major axis, and where red-giant surface activity is detected. We suggest
that when the rotational and orbital periods synchronize in close binaries, the
red-giant component is spun up, so that a dynamo mechanism starts and generates
a magnetic field, leading to observable stellar activity. Pressure modes would
then be damped as acoustic waves dissipate in these fields.
