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