3 Radial velocities

Figure 1: Small part of an UVES Echelle spectrum of ChaH$\alpha$4.

For the determination of precise RVs we used the telluric O₂ lines as wavelength reference. They are produced in the Earth atmosphere and show up in the red part of the optical spectral range (cp. Fig.1). They are stable up to $\sim$20 ms^-1 (Balthasar et al. 1982; Caccin et al. 1985).

Heliocentric RVs are determined by cross-correlating plenty of stellar lines of the object spectra against a template spectrum and locating the correlation maximum. A mean UVES spectrum of CHXR74 served as template. The zero point of its velocity has been determined by means of the Lithium line at 6708Å.

For the measurements of Doppler shifts of stellar features we carefully selected appropriate wavelength regions, which are not affected by telluric lines, cosmic defects of the CCD or fringes of the CCD in the near IR. We achieved a RV precision of $\sim$200ms^-1 for a S/N of $\sim$20 in agreement with the expectations for this S/N (Hatzes & Cochran 1992). The precision of the RVs is limited by the S/N of the spectra and not by systematic effects.

The mean heliocentric RVs are given in Table1. They are consistent with RVs measured by Neuhäuser & Comerón (1999) within the measurements uncertainties for ChaH$\alpha$1, 3, 4, 5, 7, B34 and CHXR74. However, the values for ChaH$\alpha$8 and Sz23 are discrepant by more than 1$\sigma$ and the RVs for ChaH$\alpha$2 and 6 by 2$\sigma$. This may be a hint to long-term single-lined spectroscopic binaries.

Due to a low S/N for ChaH$\alpha$7 only a mean RV and an upper limit of $v\,\sin i$ and the Lithium equivalent width could be determined.