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 < 0.3 mag 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 > 0.3 mag ) 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 3-5 \mu m , 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 20-60 d , 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 .