Keywords :
stars magnetic field - stars: low-mass, brown dwarfs - stars: rotation - techniques: spectroscopy - techniques: polarimetry
Abstract:We present in this paper, the first results of a spectropolarimetric analysis of a small sample (~20) of active stars ranging from spectral type M0 to M8, which are either fully convective or possess a very small radiative core. This study aims at providing new constraints on dynamo processes in fully convective stars. This paper focuses on ve stars of spectral type ~M4, i.e. with masses close to the full convection threshold (0.35M_{sun}_), and with short rotational periods. Tomographic imaging techniques allow us to reconstruct the surface magnetic topologies from the rotationally modulated time-series of circularly polarized profiles. We find that all stars host mainly axisymmetric large-scale poloidal fields. Three stars were observed at
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