  Free-floating substellar candidates with estimated theoretical masses of as
low as ~5 Jupiter masses have been found in the ~3 Myr old sigma Orionis open
cluster. As the overlap with the planetary mass domain increases, the question
of how these objects form becomes important. The determination of their number
density and whether a mass cut-off limit exists is crucial to understanding
their formation. We propose to search for objects of yet lower masses in the
cluster and determine the shape of the mass function at low mass. Using new-
and (re-analysed) published IZJHKs[3.6]-[8.0]-band data of an area of 840
arcmin2, we performed a search for LT-type cluster member candidates in the
magnitude range J=19.5-21.5 mag, based on their expected magnitudes and
colours. Besides recovering the T type object S Ori 70 and two other known
objects, we find three new cluster member candidates, S Ori 72-74, with J=21
mag and within 12 arcmin of the cluster centre. They have theoretical masses of
4 (-2,+3) M_Jup and are among the least massive free-floating objects detected
by direct imaging outside the Solar System. The photometry in archival Spitzer
[3.6]-[5.8]-band images infers that S Ori 72 is an L/T transition candidate and
S Ori 73 a T-type candidate, following the expected cluster sequence in the
mid-infrared. Finally, the L-type candidate S Ori 74 with lower quality
photometry is located at 11.8 arcsec (~4250 AU) of a stellar member of sigma
Orionis and could be a companion. After contaminant correction in the area
complete to J=21.1 mag, we estimate that there remain between zero and two
cluster members in the mass interval 6-4 M_Jup. Our result suggests a possible
turnover in the substellar mass spectrum below ~6 Jupiter masses, which could
be investigated further by wider and deeper photometric surveys.
