Optical spectroscopy of isolated planetary mass objects in the sigma Orionis cluster

We have obtained low resolution optical spectra of 15 isolated planetary ma ss objects (IPMOs) in the sigma Orionis cluster, and derived spectral types by comparison with nearby M and L dwarfs. The spectral types are in the ra nge late M - mid L, in agreement with our expectations based on colors and magnitudes for bona fide members. Therefore, most of these objects have mas ses below the deuterium burning limit. About 2/3 show H alpha in emission a t our spectral resolution. From our spectroscopic and photometric data, we infer that three IPMOs in this sample may be binaries with components of si milar masses. These results confirm that the substellar mass function of th e sigma Orionis cluster, in the form dN/dM, keeps rising in the planetary d omain.