Abstract:
Since 2004, we have been engaged in a long-term observing programme to monitor young
stellar objects (YSOs) in the Orion Nebula Cluster (ONC). We have collected about 2000
frames in V, R and I broad-band filters on more than 200 nights distributed over five consecutive
observing seasons. The high-quality and time-extended photometric data give us an opportunity
to address various phenomena associated with young stars. The prime motivations of this
project are (i) to explore various manifestations of stellar magnetic activity in very young
low-mass stars, (ii) to search for new pre-main-sequence eclipsing binaries and (iii) to look for
any EXor and FUor-like transient activities associated with YSOs. Since this is the first paper
on this programme, we give a detailed description of the science drivers, the observation and
the data reduction strategies as well. In addition to these, we also present a large number of
new periodic variables detected from our first 5 yr of time-series photometric data. Our study
reveals that about 72 per cent of classical T Tauri stars (CTTS) in our field of view are periodic,
whereas only 32 per cent of weak-lined T Tauri stars (WTTS) are periodic. This indicates that
inhomogeneity patterns on the surface of CTTS of the ONC stars are much more stable than
on WTTS. From our multiyear monitoring campaign, we found that the photometric surveys
based on single season are incapable of identifying all periodic variables. And any study on
evolution of angular momentum based on single-season surveys must be carried out with
caution.
