  According to dynamo theory, stars with convective envelopes efficiently
generate surface magnetic fields, which manifest as magnetic activity in the
form of starspots, faculae, flares, when their rotation period is shorter than
their convective turnover time. Most red giants, having undergone significant
spin down while expanding, have slow rotation and no spots. However, based on a
sample of about 4500 red giants observed by the NASA Kepler mission, a previous
study showed that about 8 % display spots, including about 15 % that belong to
close binary systems. Here, we shed light on a puzzling fact: for rotation
periods less than 80 days, a red giant that belongs to a close binary system
displays a photometric modulation about an order of magnitude larger than that
of a single red giant with similar rotational period and physical properties.
We investigate whether binarity leads to larger magnetic fields when tides lock
systems, or if a different spot distribution on single versus close binary
stars can explain this fact. For this, we measure the chromospheric emission in
the CaII H & K lines of 3130 of the 4465 stars studied in a previous work
thanks to the LAMOST survey. We show that red giants in a close-binary
configuration with spin-orbit resonance display significantly larger
chromospheric emission than single stars, suggesting that tidal locking leads
to larger magnetic fields at a fixed rotational period. Beyond bringing
interesting new observables to study the evolution of binary systems, this
result could be used to distinguish single versus binary red giants in
automatic pipelines based on machine learning.
