Results of the ROTOR-program - I. The long-term photometric variability of classical T Tauri stars

Context. T Tauri stars exhibit variability on all timescales, whose origin is still debated. Aims. We investigate the long term variability of CTTs over up to 20 years, characterize it from a set of statistical parameters and discuss its origin. Methods. We present a unique, homogeneous database of photometric measurements for Classical T Tauri stars extending up to 20 years. The database contains more than 21 000 UBVR observations of 72 CTTs. All the data were collected within the framework of the ROTOR-program at Mount Maidanak Observatory (Uzbekistan) and together they constitute the longest homogeneous, accurate record of TTS variability ever assembled. We characterize the long term photometric variations of 49 CTTs with sufficient data to allow a robust statistical analysis and propose an empirical classification scheme. Results. Several patterns of long term photometric variability are identified. The most common pattern, exhibited by a group of 15 stars which includes T Tau itself, consists of low level variability (Delta V <= 0.4 mag) with no significant changes occurring from season to season over many years. A related subgroup of 22 stars exhibits a similar stable long term variability pattern, though with larger amplitudes (up to Delta V similar or equal to 1.6 mag). Besides these representative groups, we identify three smaller groups of 3 - 5 stars each which have distinctive photometric properties. Conclusions. The long term variability of most CTTs is fairly stable and merely reflects shorter term variability due to cold and hot surface spots. Only a small fraction of CTTs undergo significant brightness changes on the long term (months, years), which probably arise from slowly varying circumstellar extinction.