Masses of the components of SB2 binaries observed with Gaia - I. Selection of the sample and mass ratios of 20 new SB2s discovered with Sophie

In anticipation of the Gaia astrometric mission, a large sample of spectroscopic binaries has been observed since 2010 with the Sophie spectrograph at the Haute-Provence Observatory. Our aim is to derive the orbital elements of double-lined spectroscopic binaries (SB2s) with an accuracy sufficient finally to obtain the masses of the components with relative errors as small as 1 per cent when the astrometric measurements of Gaia are taken into account. Simultaneously, the luminosities of the components in the Gaia photometric band G will also be obtained. Our observation program started with 200 SBs, including 152 systems that were only known as single-lined. Thanks to the high efficiency of the Sophie spectrograph, an additional component was found for 25 SBs. After rejection of five multiple systems, 20 new SB2s were retained, including eight binaries with an evolved primary, and their mass ratios were derived. Our final sample contains 68 SB2s, including two late-type giants and 10 other evolved stars.