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The current theory predicts that hot subdwarf binaries are produced fromevolved low-mass binaries that have undergone mass transfer and drastic massloss during either a common envelope phase or a stable Roche lobe overflowwhile on the red giant branch (RGB). We perform a spectroscopic survey to findbinary systems that include low-mass red giants near the tip of the RGB, whichare predicted to be the direct progenitors of subdwarf B (sdB) stars. We aim toobtain a homogeneous sample to search for the observational evidence ofcorrelations between the key parameters governing the formation of sdB starsand constrain the physics of stable mass transfer. In this work, weconcentrated on the southern hemisphere targets and conducted a spectroscopicsurvey of 88 red giant stars to search for the long-period RGB + MS binarysystems within 200\,pc. Combining radial velocity (RV) measurements fromground-based observations with CORALIE and RV measurements from $Gaia$ DR2 andearly data release 3 (eDR3) as well as the astrometric excess noise and RUWEmeasurements from $Gaia$ DR3, we defined a robust binary classification method.In addition, we searched for known binary systems in the literature and in the$Gaia$ DR3. We select a total of 211 RGB candidates in the southern hemispherewithin 200\,pc based on the $Gaia$ DR2 color-magnitude diagram. Among them, atotal of 33 red giants were reported as binary systems with orbital periodsbetween 100 and 900 days, some of which are expected to be the directprogenitors of wide binary sdB stars. In addition, we classified 37 newMS\,+\,RGB binary candidates, whose orbital parameters will be measured withfuture spectroscopic follow-up.

Volume-limited sample of low-mass red giant stars, the progenitors of hot subdwarf stars I. Sample selection and binary classification method