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Context. Brown dwarf companions to stars at separations of a few AU or lessare rare objects, and none have been found so far around early-type M dwarfsM0V-M5V). With GJ 1046 (M2.5V), a strong candidate for such a system with aseparation of 0.42 AU is presented.  Aims. We aim at constraining the mass of the companion in order to decidewhether it is a brown dwarf or a low-mass star.  Methods. We employed precision RV measurements to determine the orbitalparameters and the minimum companion mass. We then derived an upper limit tothe companion mass from the lack of disturbances of the RV measurements by asecondary spectrum. An even tighter upper limit is subsequently established bycombining the RV-derived orbital parameters with the recent new version of theHipparcos Intermediate Astrometric Data.  Results. For the mass of the companion, we derive m>26.9 MJup from the RVdata. Based on the RV data alone, the probability that the companion exceedsthe stellar mass threshold is just 6.2%. The absence of effects from thesecondary spectrum lets us constrain the companion mass to m <229 MJup. Thecombination of RV and Hipparcos data yields a 3sigma upper mass limit to thecompanion mass of 112 MJup with a formal optimum value at m=47.2 MJup. From thecombination of RV and astrometric data, the chance probability that thecompanion is a star is 2.9%.  Conclusions. We have found a low-mass, close companion to an early-type Mdwarf. While the most likely interpretation of this object is that it is abrown dwarf, a low-mass stellar companion is not fully excluded.

A probable close brown dwarf companion to GJ 1046 (M2.5V)