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T Tauri stars exhibit variability on all timescales, whose origin is stilldebated. 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 anddiscuss its implications for our understanding of these stars. We have obtaineda unique, homogeneous database of photometric measurements for WTTS extendingup to 20 years. It contains more than 9 000 UBV R observations of 48 WTTS. Allthe data were collected at Mount Maidanak Observatory (Uzbekistan) and theyconstitute the longest homogeneous record of accurate WTTS photometry everassembled. Definitive rotation periods for 35 of the 48 stars are obtained.Phased light curves over 5 to 20 seasons are now available for analysis. Lightcurve shapes, amplitudes and colour variations are obtained for this sample andvarious behaviors exhibited, discussed and interpreted. Our main conclusion isthat most WTTS have very stable long term variability with relatively smallchanges of amplitude or mean light level. The long term variability seenreflects modulation in the cold spot distributions. Photometric periods arestable over many years, and the phase of minimum light can be stable as wellfor 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.

Results of the ROTOR-program. II. The long-term photometric variability of weak-line T Tauri stars