Rains, AdamZerjal, MarusaIreland, MichaelNordlander, ThomasBessell, MikeCasagrande, LucaOnken, ChristopherJoyce, MeridithKammerer, JensAbbot, Harrison2024-03-052024-03-050035-8711http://hdl.handle.net/1885/315717We present the results of a medium-resolution optical spectroscopic survey of 92 cool ($3000 \,\mathrm{ K}\lesssim T_{\rm eff} \lesssim 4500\,$K) southern TESS candidate planet hosts, and describe our spectral fitting methodology used to recover stellar parameters. We quantify model deficiencies at predicting optical fluxes, and while our technique works well for Teff, further improvements are needed for [Fe/H]. To this end, we developed an updated photometric [Fe/H] calibration for isolated main-sequence stars built upon a calibration sample of 69 cool dwarfs in binary systems, precise to $\pm 0.19\,$dex, from supersolar to metal poor, over 1.51 < Gaia (BP - RP) < 3.3. Our fitted Teff and R⋆ have median precisions of 0.8 per cent and 1.7 per cent, respectively, and are consistent with our sample of standard stars. We use these to model the transit light curves and determine exoplanet radii for 100 candidate planets to 3.5 per cent precision and see evidence that the planet radius gap is also present for cool dwarfs. Our results are consistent with the sample of confirmed TESS planets, with this survey representing one of the largest uniform analyses of cool TESS candidate planet hosts to date.ADR acknowledges support from the Australian Government Research Training Program, and the Research School of Astronomy & Astrophysics top-up scholarship. MZ and MJI acknowledge ˇ funding from the Australian Research Council (grant DP170102233). LC is the recipient of the ARC Future Fellowship FT160100402. MJ was supported the Research School of Astronomy and Astrophysics at the Australian National University and funding from Australian Research Council grant no. DP150100250. Parts of this research were conducted by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013.application/pdfen-AU© 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Societytechniques: spectroscopicplanets and satellites: fundamental parametersstars: fundamental parametersstars: low-mass202110.1093/mnras/stab11672022-10-16