2 General properties of the object

RX J1603.8-3938 was first detected as a strong X-ray source by ROSAT (Krautter et al. 1997). Optical follow up observations by Wichmann et al. (1997) then demonstrated that the object is a pre-main sequence star. The binary nature was first recognised on a single high resolution spectrum taken with CASPEC on the ESO 3.6 m telescope (Wichmann et al. 1999). Based on U B V R I-photometry, and the fact that the equivalent width of H$\alpha$ is less than 10 Å, RX J1603.8-3938 is classified as weak-line TTauri star. This classification is in agreement with the equivalent width of the LiI 6708-line of the two components of $0.41\pm0.04$ Å, and $0.43\pm0.07$ Å (see Sect. 4.2).

RX J1603.8-3938 was also observed by the HIPPARCOS satellite, and is included in the TYCHO-2 catalogue (TYC 7855-01106-1). Using the formula given in the TYCHO catalogue we converted the $B_{\rm T}$ and $V_{\rm T}$ magnitudes to the Johnson system. We find that the V-magnitude of $11.01\pm0.17$ and the B-V colour index of $0.97\pm0.16$are fully consistent with the V-magnitude of 11.02 and B-V colour index 1.05 obtained by Wichmann et al. (1997). The object is located in the Lupus 3 star forming region.

The proper motion of $-16.4\pm2.4$ $\rm mas\,yr^{-1}$ in RA, and $-28.4\pm2.3$ $\rm mas\,yr^{-1}$ in DEC as listed in the TYCHO-2 Catalogue is also consistent with membership in the Lupus star-forming region. The generally accepted distance to the Lupus 3 star-forming region is about 140 pc (Hughes et al. 1997). However, this value has recently been questioned, as Knude & Hog (1997) derived a distance of only 100 pc.