$\sim$M9$-$L0, their low surface gravities (L06; W09), and their low luminosities. Low gravity was determined by comparing these objects’ H and K band pseudo-continuum profiles with those of other low gravity brown dwarfs and high gravity field dwarfs in low resolution spectra. These features confirmed that these objects were cluster members, and therefore that their ages are unlikely to be more than $\sim$10 Myr. The PMOs are within a few arcminutes of the centre of the ONC, so they are likely to be very young ($\sim$1 Myr) objects, rather than members of the more dispersed and slightly older population that extends over a few degrees on the sky.

In the event, useful observations were obtained for only one ONC PMO, 152$-$717. This source has apparent magnitude K=17.6, which is typical of the sample in L06. It was not possible to obtain a guide star lock for the other two PMOs, owing to the bright background of the nebulosity. We note that this observing method could be more successfully employed in future when the patrol radius of the Gemini Altair AO system has been increased, allowing the use of brighter guide stars.

The calibrators consisted of two field dwarfs, 2MASS 0345+25, 