2006A&A...450.1135B

The ROSAT All-Sky Survey detected many young objects outside any known star forming region. Their formation is yet unclear. In order to improve the knowledge about these X-ray bright objects we aimed at measuring their rotational properties, which are fundamental stellar parameters, and at comparing them to young objects inside molecular clouds. We monitored photometric variations of 5 T Tauri stars in MBM12 and of 26 young objects in the Taurus-Auriga molecular cloud and south of it. Among the 26 young objects there are 17 weak-line T Tauri stars, 7 zero age main-sequence stars and 2 of unknown type. In addition, 2 main-sequence K-type stars were observed, and one comparison star turned out to be an eclipsing binary. We found periodic variations for most of the targets. The measured periods of the T Tauri stars range from 0.57 to 7.4-days. The photometric variation can be ascribed to rotational modulation caused by spots. For a few of the periodic variables, changes of the light curve profile within several weeks are reported. For one star such changes have been observed in data taken two years apart. The exceptions are two eclipsing systems. One so far unknown system - GSC2.2N3022313162 - shows a light curve with full phase coverage having both primary and secondary minima well resolved. It has an orbital period of 0.59075-days. From our spectroscopic observations we conclude that it is a main sequence star of spectral type F2±4. We further compared the off-cloud weak-line T Tauri stars to the weak-line T Tauri stars inside the molecular cloud in terms of rotational period distribution. Statistical analysis of the two samples shows that both groups are likely to have the same period distribution.