  We explore UV and optical variability signatures for several hundred members
of NGC 2264 (3 Myr). We performed simultaneous u- and r-band monitoring over
two full weeks with CFHT/MegaCam. About 750 young stars are probed; 40% of them
are accreting. Statistically distinct variability properties are observed for
accreting and non-accreting cluster members. The accretors exhibit a
significantly higher level of variability than the non-accretors, especially in
the UV. The amount of u-band variability correlates statistically with UV
excess in disk-bearing objects, which suggests that accretion and star-disk
interaction are the main sources of variability. Cool magnetic spots, several
hundred degrees colder than the photosphere and covering from 5 to 30% of the
stellar surface, appear to be the leading factor of variability for the
non-accreting stars. In contrast, accretion spots, a few thousand degrees
hotter than the photosphere and covering a few percent of the stellar surface,
best reproduce the variability of accreting objects. The color behavior is also
found to be different between accreting and non-accreting stars. Typical
variability amplitudes for accreting members rapidly increase from r to u,
which indicates a much stronger contrast at short wavelengths; a lower color
dependence in the amplitudes is instead measured for diskless stars. We find
that u-band variability on hour timescales is typically about 10% of the
peak-to-peak variability on day timescales, while longer term (years)
variability is consistent with amplitudes measured over weeks. We conclude that
for both accreting and non-accreting stars, the mid-term rotational modulation
by spots is the leading timescale for a variability of up to several years. In
turn, this suggests that the accretion process is essentially stable over
years, although it exhibits low-level shorter term variations in single
accretion events.
