R. Silvotti, INAF-Osservatorio Astrofisico di Torino Single vs binary hot subdwarfs: towards a 200-pc volume-limited complete sample Abstract From current statistics, based on mag-limited samples, 35% of sdO/B hot subdwarfs are in close binaries with M-dwarf/WD companions, ~30% are in wide binaries with F/G/K MS stars, 35% are apparently singles. However, without a companion it is hard to explain the huge mass loss near the RGB tip needed to form an sdB star. The presence of a substellar companion, difficult to detect, is a possibility. The goal of this project is to shed light on the formation mechanisms of single sdO/B stars by searching for binary signatures on a small but complete 200-pc volume-limited sample using different data/methods: TESS light curves, Gaia spectra, SEDs, and precise RVs from Harps/Harps-N spectra. Introduction, selection of the targets, data/methods The target list was extracted from the catalog of Culpan+2022 (A&A 662, A40), based on Gaia EDR3, which was complete up to about 250 pc as we can see from Fig.1. As a first step we considered the sample within 200 pc (20 stars) and later we added one missing star from the more recent 500-pc volume-limited sample by Dawson+2024 (A&A 686, A25). These 21 stars are listed in Table 1. We can see that 2 are in tight binaries with a WD, 1 is in a tight binary with an M-dwarf, and 3 have MS companions in larger orbits. Among the remaining 15 stars, 3 show tiny periodic variations in their TESS light curves, and one shows irregular photometric variations. The objective of this project is to verify how many of these stars are truly single, moving down as much as possible the detection limit of a faint, low-mass companion. The HARPS and HARPS-N instruments are ideal to obtain a homogeneous set of high- resolution spectra in both hemispheres, from which accurate radial velocities (RVs) can be derived. Using the cross correlation function (CCF) on 100-200 metal absorption lines (up to 500 in one case), we can reach typical errors of 100-200 m/s (Fig.2). Preliminary results and short discussion At least 6 out of 15 apparently single sdO/B stars within 200 pc do not show TESS photometric variations nor RV variations within measurement limits. Three stars show short-period variations in their TESS light curves, presumably due to a companion, which may or may not be confirmed by ongoing RV measurements (an example is given in Fig.3). One star shows irregular photometric variations in its TESS light curve and it should be observed soon with HARPS@3.6m. Two stars (PG0342+026 and PG0044+097) are known g-mode pulsators (Sahoo+2020, MNRAS 495, 2844; Silvotti+2020, MNRAS 489, 4791) and for this reason their RV data are not shown in Fig.2. These 2 articles report some limits on the presence of a companion, however the RV variations caused by g-mode pulsations make it more difficult to set reliable strong limits. Finally, 3 stars do not have yet RV measurements: one has only 1 spectrum but should be reobserved this summer with HARPS-N@TNG, another should be observed this autumn with HARPS@3.6m, the third does not yet have scheduled observations. A few stars show a weak flux excess near 400 nm in their Gaia BP/RP XP spectrum that might be due to a WD companion but further analyses (and additional RV data) are needed to confirm this finding. Fig.2. Radial velocities obtained from Harps-N spectra using the CCF method with 4 masks, corresponding to 4 different models: 25000 K He-poor, 35000 K He-poor, 38000 K He-rich, 50000 K, He-rich. The masks were built from 4 line lists kindly provided by Matti Dorsch. Fig.3. Upper panel: TESS Fourier spectrum of one of the targets showing periodic photometric variations (top) and phase diagram of the RV measurements folded onto the main photometric period at 0.315 d (bottom). These RV measurements are not yet sufficient to confirm the presence of a low-mass companion (they are compatible with constant RVs) and more data will be needed. Lower p