  The extremely young cluster IC 348 has been monitored photometrically over 5
observing seasons from Dec 1998 to March 2003 in Cousins I with a 0.6 m
telescope at Van Vleck Observatory. Twenty-eight periodic variables and 16
irregular variables have been identified. Among the brighter stars, 14 of the
16 known K or M-type WTTS were found to be periodic variables, while all 5 of
the known CTTS were found to be irregular variables. In the full sample, which
includes 150 stars with I mag as faint as 18, we find that 40% of the 63 WTTS
are detected as variables, nearly all of them periodic, while 55% of the 20
CTTS are also detected as variable, with none of them periodic. Our study
suggests that 80-90% of all WTTS in young clusters will be detected as periodic
variables given sufficiently precise and extended monitoring, whereas CTTS will
reveal themselves primarily or solely as irregular variables. This has clear
consequences for PMS rotational studies based on photometric periods. We
examine the stability of the periodic light curves from season to season. All
periodic stars show modulations of their amplitude, mean brightness and light
curve shape on time scales of less than 1 yr, presumably due to changes in spot
configurations and/or physical characteristics. In no case, however, can we
find definitive evidence of a change in period, indicating that differential
rotation is probably much less on WTTS than it is on the Sun. Among the
non-periodic stars, we report the detection of two possible UXors as well as a
pre-main sequence star, HMW 15, which apparently undergoes an eclipse with a
duration exceeding three years.
