  T Tauri stars exhibit variability on all timescales, whose origin is still
debated. On WTTS the variability is fairly simple and attributed to long-lived,
ubiquitous cool spots. We investigate the long term variability of WTTS,
extending up to 20 years in some cases, characterize it statistically and
discuss its implications for our understanding of these stars. We have obtained
a unique, homogeneous database of photometric measurements for WTTS extending
up to 20 years. It contains more than 9 000 UBV R observations of 48 WTTS. All
the data were collected at Mount Maidanak Observatory (Uzbekistan) and they
constitute the longest homogeneous record of accurate WTTS photometry ever
assembled. Definitive rotation periods for 35 of the 48 stars are obtained.
Phased light curves over 5 to 20 seasons are now available for analysis. Light
curve shapes, amplitudes and colour variations are obtained for this sample and
various behaviors exhibited, discussed and interpreted. Our main conclusion is
that most WTTS have very stable long term variability with relatively small
changes of amplitude or mean light level. The long term variability seen
reflects modulation in the cold spot distributions. Photometric periods are
stable over many years, and the phase of minimum light can be stable as well
for several years. On the long term, spot properties do change in subtle ways,
leading to secular variations in the shape and amplitudes of the light curves.
