Spectral type of the objects as a function of the spectral index values obtained with different NIR indices (Table B.1). Red symbols are values from this work, while green crosses are from Testi et al. (2001) and Testi (2009). Red stars are used for objects with SpT in the range of validity of the index, while red pentagons for those not in the range of validity of the index. Dashed lines are the best fit from the reference cited in Table B.1 for each index, when available. No published relations are available for the indices I_J and I_K. Black and red solid lines are the best fits (black = linear, red = polynomial) from this work, considering both our sample and the values from the literature.