  We present a reference study of the long-term optical variability of young
stars using data from the WASP project. Our primary sample is a group of
well-studied classical T Tauri stars (CTTS), mostly in Taurus-Auriga. WASP
lightcurves cover timescales up to 7 years and typically contain 10000-30000
datapoints. We quantify the variability as function of timescale using the
time-dependent standard deviation 'pooled sigma'. We find that the overwhelming
majority of CTTS has low-level variability with sigma<0.3mag dominated by
timescales of a few weeks, consistent with rotational modulation. Thus, for
most young stars monitoring over a month is sufficient to constrain the total
amount of variability over timescales up to a decade. The fraction of stars
with strong optical variability (sigma>0.3mag) is 21% in our sample and 21% in
an unbiased control sample. An even smaller fraction (13% in our sample, 6% in
the control) show evidence for an increase in variability amplitude as a
function of timescale from weeks to months or years. The presence of long-term
variability correlates with the spectral slope at 3-5mu, which is an indicator
of inner disk geometry, and with the U-B band slope, which is an accretion
diagnostics. This shows that the long-term variations in CTTS are predominantly
driven by processes in the inner disk and in the accretion zone. Four of the
stars with long-term variations show periods of 20-60d, significantly longer
than the rotation periods and stable over months to years. One possible
explanation are cyclic changes in the interaction between the disk and the
stellar magnetic field.
