Colour−magnitude diagram of sources in N159W detected in only the H- and K_s-bands (black points) and those in the control field (black points with green circles). Also plotted are the zero-age main sequence (solid blue lines) and the 1 Myr PMS isochrone from Siess et al. (2000, dotted red lines) for extinctions of A_V = 0 and 15 mag. Red circles highlight candidate YSOs identified from our colour cut (see text for details). Blue circles highlight candidate YSOs previously identified from the CCD (Fig. 5).