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Aims: We investigated in detail nine sources in the direction of the youngsigma Orionis cluster, which is considered a unique site for studying stellarand substellar formation. The nine sources were selected because of somepeculiar properties, such as extremely red infrared colours or too strongHalpha emission for their blue optical colours. Methods: We took high-quality,low-resolution spectroscopy (R ~ 500) of the nine targets with ALFOSC at theNordic Optical Telescope. We also re-analyzed [24]-band photometry fromMIPS/Spitzer and compiled the best photometry available at the ViJHKs passbandsand the four IRAC/Spitzer channels for constructing accurate spectral energydistributions covering from 0.55 to 24 mum. Results: The nine targets wereclassified into: one Herbig Ae/Be star with a scatterer edge-on disc, twoG-type stars, one X-ray flaring, early-M, young star with chromospheric Halphaemission, one very low-mass, accreting, young spectroscopic binary, two youngobjects at the brown dwarf boundary with the characteristics of classical TTauri stars, and two emission-line galaxies, one undergoing star formation, andanother one whose spectral energy distribution is dominated by an activegalactic nucleus. Besides, we discover three infrared sources associated tooverdensities in a cold cloud in the cluster centre. Conclusions:Low-resolution spectroscopy and spectral energy distributions are a vital toolfor measuring the physical properties and the evolution of young stars andcandidates in the sigma Orionis cluster.

Low-resolution spectroscopy and spectral energy distributions of selected sources towards sigma Orionis