A DECADE OF PHOTOMETRIC-OBSERVATIONS OF YOUNG STARS - WITH SPECIAL COMMENTS ON PERIODICITIES

We have collected photometric observations of 16 young stellar objects during 10 observing periods from 1981 to 1991. The photometry include s measurements with broad-band Stromgren uvby and standard UBVRIJHKL i ntermediate filters in addition to narrow-band Halpha and Hbeta filter s. A large part of the material has been discussed elsewhere and the p urpose of the present article is to publish all individual measurement s, to comment on light curves and colour diagrams not presented before and to make a search for periodic light fluctuations. The material in cludes 8 classical T Tauri stars (CTTS), 5 weak-line T Tauri stars (WT TS), 2 post T Tauri candidates (PTTS) and 1 Herbig Ae star. We have se arched for periodic light variations in 15 of the stars, of which 7 ha ve not been followed photometrically before. None of the CTTS shows di stinct periods. On this basis and a literature survey published elsewh ere we conclude that many announced periods are still doubtful. Many o f these stars tend to brighten and fade on certain time-scales which c an differ from time to time. We also arrive at the conclusion that nev ertheless about 1/4 of the CTTS studied so far have well defined perio ds. All five WTTS show evidence of regular fluctuations of small ampli tudes on time-scales of a few days but for 3 stars (San 1 Andrew 481, SZ Cha) the data base is too small for any definite conclusions to be drawn. The amplitudes of the variations increase with decreasing wavel ength and rotational modulation of dark spots on the stellar surface i s consistent with such variations. In this picture Sz 82 (The 12), wit h a period of 1.2 days, is a fast rotator with pronounced ultraviolet activity and X-ray emission. RY Lup is a well studied case with an est ablished period of 3.75 days. It is a special case of WTTS since it ca rries a large infrared excess. One PTTS was followed over several nigh ts but shows no evidence of periodicity. For the Herbig Ae star AB Aur no period can be announced at present even though one Fourier method based on CLEAN does indicate periods close to what has been found from spectral line variability.