3 Observations and data-reduction

Using the Echelle spectrograph FEROS (Fiber fed Extended Range Spectrograph) on the 1.5 m telescope at La Silla, we have obtained 17 spectra of RX J1603.8-3938 over a time span of about one and a half years. The spectra cover the wavelength region between about 3600 Å and 9200 Å, and have a resolution of $\rm\lambda / \Delta \lambda=48\,000$. FEROS is optimised for high precision measurements of the radial velocities. The standard MIDAS pipeline for FEROS was used to subtract bias, flat-field, remove the scattered light, subtract the sky background, and extract and wavelength calibrate the spectra. Precise measurements of the radial-velocity have then been carried out by cross-correlating the spectra with spectra of the radial-velocity standard HR5777 which was observed with FEROS too. The radial velocity of HR5777 is known to be stable, and has been determined with an accuracy of $\rm 55$ ms^-1 (Murdoch 1993). We have observed HR5777 in every observing run and find that the accuracy of our measurements is better than 80 ms^-1. However, because the signal to noise ratio is less in RX J1603.8-3938, the errors are of the order of 250 ms^-1 for the primary component, and somewhat larger for the secondary component.

Figure 1: Orbital solution for RX J1603.8-3938 from the elements given in Table 2. The scatter around the fit is 0.54 and 0.57 km-1 for the primary (filled symbols) and secondary (open symbols)