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 (∆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 ∆V 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.

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A new photometric and spectroscopic survey of the star formation region (SFR) CMa R1 is described. In a sample of 165 stars brighter than 13th mag, 88 stars were found to be probable members of the SFR. They are defined as early‐type stars with E(B−V)0.16 mag, which corresponds to a distance of about 1 kpc. 74 of the probable members are B stars. 19 stars are possibly associated with an IRAS point source. We derive a most probable distance of 1050±150 pc to the association. It appears that about 80 candidate members are pre‐main‐sequence stars with ages lower than 6 million years, while the main sequence extends over 6.0–7.6 mag, which is consistent with star formation starting about 8 million years ago and continuing until at least half a million years ago. Two bright B stars in the association (GU CMa and FZ CMa) seem to be much older and probably do not originate from the same star formation episode. The star formation efficiency appears to increase roughly monotonically with time up to half a million years ago. From our data, we conclude that only a minor fraction of the stars has been created through the scenario suggested by Herbst & Assousa, in which the members of CMa R1 form by compression of ambient material by a supernova shock wave. An extensive search for candidate members with Hα emission did not reveal new Herbig Ae/Be candidates, so that the number of stars in this class seems to be limited to four: Z CMa, LkHα 218, LkHα 220 and possibly HD 53367.

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We report some results of a long-term photometric program called ROTOR being carried out at Maidanak Observatory in the Republic of Uzbekistan. The goal of the program is to search for periodic components in the light of T Tauri stars (TTSs), Herbig Ae/Be stars (HAEBEs), FU Orionis stars, and related objects. In this paper we analyze results obtained for TTSs in the vicinity of the o Oph dark clouds obtained between 1986 and 1993. Our sample includes 6 weak-emission TTSs (WTTSs) and 11 classical TTSs (CTTSs). All of the WTTSs investigated have signiÐcant photometric periods that are attributed to spottedness of the stellar photospheres. Two of them, SR 9 and SR 12, maintained stable periods and phases during the entire span of observation. The periods are between 1 and 4 days, except for SR 9, which has a period of 6.53 days ; its relatively large Ha equivalent width (for a WTTS) also suggests it may be a transition object between the WTTSs and CTTSs. Among the CTTSs in our sample, we found only three stars with deÐnite periods, two of which were longer than 8 days. Generally speaking, therefore, our observations are in accordance with the canonical view of WTTSs and CTTSs, in which stars with more active accretion disks spin slower due to disk locking and show greater irregular variations due to accretion hot spots. The exception is V895 Sco (Haro 1-1), which seems to be a typical CTTS with a rotation period of less than 4 days. The stability of the periods and epochs of minima of some or most WTTSs is noted and discussed.