2010MNRAS.406..505S

Identifying the population of young stellar objects (YSOs) in high-extinction regions is a pre-requisite for studies of star formation. This task is not trivial, as reddened background objects can be indistinguishable from YSOs in near-infrared colour-colour diagrams. Here we combine deep JHK photometry with J- and K-band light curves, obtained with the United Kingdom Infrared Telescope, to explore the YSO population in the dark cloud IC1396W. We demonstrate that a colour-variability criterion can provide useful constraints on the star-forming activity in embedded regions. For IC1396W we find that a near-infrared colour analysis alone vastly overestimates the number of YSOs. In total, the globule probably harbours not more than 10 YSOs, among them a system of two young stars embedded in a small (∼10000au) reflection nebula. This translates into a star-forming efficiency (SFE) of ∼1 per cent, which is low compared with nearby more massive star-forming regions, but similar to less massive globules. We confirm that IC1396W is likely associated with the IC1396 HII region. One possible explanation for the low SFE is the relatively large distance to the ionizing O star in the central part of IC1396. Serendipitously, our variability campaign yields two new eclipsing binaries, and eight periodic variables, most of them with the characteristics of contact binaries.