7.5.1 Introduction

The non-single stars processing is intended to perform the reduction of the corresponding data from the point of view of multiple star studies. For the brightest stars, the RVS instrument provides spectra that are used to derive physical parameters and radial velocities for stars exhibiting a normal single spectrum. The radial velocity is measured by cross-correlating the observed reduced spectra with a theoretical/synthetic spectrum computed with various stellar atmosphere models. In addition to the various methods measuring the radial velocities as done in the STA (Single Transit Analysis) pipeline, a particular code is trying to detect composite spectra. If a spectra is suspected to be composite, it is further processed to confirm/infirm the double character of the observed spectrum and to measure both radial velocities. The approach used for Gaia is based on a bi-dimensional extension of the cross-correlation function being premised on the TodCor method (Zucker and Mazeh 1994). The objects presenting confirmed composite spectra are deviated from the subsequent main spectroscopic processing (see Chapter 6) after the measurements of both RVs and both projected rotational velocities. This is done transit per transit. The list of objects being spectroscopically double (SB2) is transmitted to the NSS processing along with the two radial velocities and the two projected broadening velocities.