Discovery of a very cool object with extraordinarily strong Hα emission

We report on the finding of the strongest Halpha emission - pseudoequivalent width of 705 Angstrom - known so far in a young, late type dwarf. This object, named as S Ori 71, is a substellar candidate member of the 1-8 Myr star cluster sigma Orionis. Due to its overluminous location in color-magnitude diagrams, S Ori 71 might be younger than other cluster members, or a binary of similar components. Its mass is in the range 0.021-0.012 M-., depending on evolutionary models and possible binarity. The broad H line of S Ori 71 appears asymmetric, indicative of high velocity mass motions in the Halpha forming region. The origin of this emission is unclear at the present time. We discuss three possible scenarios: accretion from a disk, mass exchange between the components of a binary system, and emission from a chromosphere.