T-CHAMAELEONTIS - A WEAK-LINE YY ORIONIS STAR

We present new low and high resolution spectroscopy and optical photom etry of the star T Cha. From the low resolution spectroscopy we classi fy T Cha as a G8 type star. We also identify T Cha as the optical coun terpart of the ROSAT all-sky survey X-ray source RX J1157.2-7921. On t he basis of the physical association with the Chamaeleon cloud complex , the erratic photometric variability and colour behaviour, the presen ce of Halpha emission and of strong Li lambda 6708 absorption line, as well as its position on the HR diagram, we establish unambiguously th e low-mass PMS nature of the star. These characteristics, combined wit h an observed equivalent width of the Halpha emission less than 10 ang strom, suggest that the star can be classified as a weak-line T Tauri star. However, the strong IR excess of the star, resembling more close ly those of classical T Tauri stars, clearly indicates the presence of a circumstellar disk. From spectra obtained at different epochs we fi nd that strong variability is present in the Halpha line, with the pro file changing from pure emission to an inverse P Cygni on a time scale of one day or less. Actually, T Cha is the first case in which an inv erse P Cygni profile is observed at Halpha. Hence, here we suggest tha t T Cha can be considered a case of ''weak-line'' YY Orionis star.