  Using the 8k x 8k CCD Mosaic Camera on the 1m Schmidt telescope in Venezuela,
we are conducting a large-scale, deep optical, multiepoch, photometric (BVRIHa)
survey over 120 sq.deg. in the Orion OB association, aimed at identifying the
low mass stellar populations with ages less than about 10 Myr.
  We present initial results for a 34 sq.deg. area spanning Orion 1b, 1a and
the B Cloud. Using variability as our main selection criterion we derive much
cleaner samples than with the usual single-epoch photometric selection,
allowing us to attain a much higher efficiency in follow up spectroscopy and
resulting in an preliminary list of 74 new low-mass (~ 0.4 Msun) pre-main
sequence stars.
  Though preliminary, this list of new T Tauri stars already suggests that the
fraction of accreting young stars in 1a is much lower than in 1b, which would
be expected if 1a is indeed older than 1b. We are analyzing in detail the light
curves of these new stars, and spectroscopy of further candidates is under way.
