New weak-line T Tauri stars in Orion from the ROSAT all-sky survey

We present results of the spectroscopic and photometric follow-up observations of the ROSAT all-sky survey in the direction of the Orion cloud complex. The main goal of these observations is the search for X-ray emitting pre-main sequence stars. 820 X-ray sources were detected with high confidence in about 450 square degrees. The mean density of X-ray sources in this region is a factor of about two higher than that of the whole RASS. 5% of the RASS sources in this region are identified with previously known and likely pre-main sequence stars. We have investigated spectroscopically 181 new RASS sources widely distributed over the entire cloud complex. On the basis of the presence of strong Li I lambda 6707 absorption, spectral type later than F0 and chromospheric emission, 112 new weak-line T Tauri stars could be found. We present coordinates, X-ray count-rates and finding charts of the new PMS. Optical UBV(RI)(KC), near-infrared JHKLM and uvby-beta photometry for the new WTTS is also provided. In addition 24 dKe-dMe stars were also found on the basis of the RASS data.