H$_{2}$(K) index as age indicators.

Section 10 contains our conclusions.

Selection of Targets
====================

In the last few years, large populations of brown dwarfs have been discovered in several very young clusters where the mass function appears to extend below the deuterium-burning threshold of 0.012-0.013M$_{\odot}$ (@lucas00; @zapatero00; @scholz11; @scholz12). The ONC contains the largest known sample of very young brown dwarfs, and therefore is an ideal site to obtain good statistics on the IMF at planetary masses and to gain insights into low-mass brown dwarf formation processes. The formation mechanism responsible for the low masses of the substellar objects is currently unknown.

The three ONC PMOs are 183$-$729 (18), 152$-$717 (27) and 137$-$532 (172) (@lucas00; @lucas05, hereafter LRT05) (the numbers in brackets are the source catalogue numbers quoted in LRT05). They were selected because each has a bright star $I_{MAG}$=15$-$16 and $R_{MAG}$=17$-$18 within $\sim$25$\arcsec$ that could potentially be used as a tip/tilt guide star for laser-guided adaptive optics (AO) and they are bright enough for medium resolution ground based spectroscopy. We note that there is only a modest range of apparent K magnitudes amongst the spectroscopically confirmed PMOs in L06. Their PMO status is based on their spectral types of 