Since the discovery 30 years ago of the signature of the solar 11-year activity cycle in the eigenfrequencies of the acoustic oscillations, continuous helioseismic observations from ground-based networks (BiSON, IRIS) and space-based instruments (GOLF, VIRGO) have revealed a strong correlation between the phases of the cycle and the mode properties. Oscillation frequencies and amplitudes are anti-correlated and follow the magnetic activity proxies (e.g. UV, spots). Unlike other standard indices, temporal fluctuations of the acoustic parameters make it possible to probe the structural and magnetic changes in the subsurface layers. These effects are however minimal (~ 0.01%) and such studies have been long limited to the Sun. Since the advent of asteroseismic space missions like CoRoT and Kepler, this accuracy is now possible to achieve for other stars, which opens the door for the study of stellar magnetic cycles through seismology. This thematic is emerging and it will soon take an important place bringing together disciplines such as asteroseismology and spectropolarimetry.

The purpose of this 2-day workshop is to review the knowledge acquired for the Sun and the recent first results obtained for stars. It will aim to bring theorists and observers together to understand the differences between stellar and solar cycles, while considering also the preparation for future missions such as PLATO.

 Solar and stellar cycles: principle, differences.

 Solar oscillations over the 3 last cycles (BiSON, GOLF, VIRGO).

 Correlations between oscillations and standard magnetic activity proxies.

 Theory: dynamo modelisation, impacts on the oscillations.

 First confirmed detections of stellar cycles with seismology.

 Current spectroscopic surveys and differences with the spectral type.

 Links activity-rotation-age.

Status on spectro-polarimetry: knowledge, targets.

 Prospective : Probe magnetic activity with SONG, PLATO. Complementarities.

Lionel Bigot (Chair), Orlagh Creevey, Rafael Garcia, David Salabert, Isabelle Lapassat