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We present the spectroscopic characterization of the unusual high-amplitudevery low mass pre-main-sequence periodic variable CHS7797. This study is basedon optical medium-resolution (R=2200) spectroscopy in the 6450-8600 A range,carried out with GMOS-GEMINI-S in March 2011. Observations of CHS7797 have beencarried out at two distinct phases of the 17.8d period, namely at maximum andfour days before maximum. Four different spectral indices were used for thespectral classification at these two phases, all of them well-suited forspectral classification of young and obscured late M dwarfs. In addition, thegravity-sensitive NaI (8183/8195 A) and KI (7665/7699 A) doublet lines wereused to confirm the young age of CHS7797. From the spectrum obtained at maximumlight we derived a spectral type (SpT) of M6.05, while for the spectrum takenfour days before maximum the derived SpT is M5.75. The derived SpTs confirmthat CHS7797 has a mass in the stellar-substellar boundary mass range. Inaddition, the small differences in the derived SpTs at the two observed phasesmay provide indirect hints that CHS7797 is a binary system of similar masscomponents surrounded by a tilted circumbinary disk, a system similar to KH15D.

The spectral type of CHS7797 - an intriguing very low mass periodic variable in the Orion Nebula Cluster