  The nearby young sigma Orionis cluster (~360 pc, ~3 Ma) is becoming one of
the most important regions for the study of ultra low-mass star formation and
its extension down to the mass regimes of the brown dwarfs and planetary-mass
objects. Here, I introduce the sigma Orionis cluster and present three studies
that the JOVIAN group is developing: a pilot programme of near-infrared
adaptive-optics imaging, intermediate-resolution optical spectroscopy of a
large sample of stars of the cluster and a study of the mass function down to
the planetary-mass domain. This paper is a summary of the content of four
posters that I presented in the Ultra low-mass star formation and evolution
Workshop, as single author or on behalf of different collaborations.
