determine the signal. The photon noise was then calculated using Poisson statistics.[^2]

Our sample consists of objects in two known age groups, $\sim$1-2 Myr, $\sim$3-7 Myr, and those in the field. The age of the field objects is unknown, but they are significantly older than the objects in the known age groups. It would be interesting to examine the pEWs of gravity-sensitive features in objects with ages of a few 100 Myr. For example, the Pleiades cluster ($\sim$120 Myr) is known to contain a number of late M/early L dwarfs (@bihain10, hereafter B10). (Note that later in this paper we examine an extended dataset, in which we derive the H$_{2}$(K) indices of several Pleiades objects.) We examined whether the CaI triplet and the CO trough at 2.29$\mu$m are also affected by surface gravity. For both these features, the pEWs of the two field dwarfs are larger than those of the other objects in our sample, particularly in the case of the CO trough, while the pEWs for these features among the two sets of younger calibrators are broadly similar.

![NaI pEW as a function of age for our brown dwarf calibrators. The red diamonds are the mean pEWs of the 