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Accurate mass and radius determinations of a cool subdwarf in an eclipsing binary


Cool subdwarfs are metal-poor low-mass stars that formed during the early stages of the evolution of our Galaxy. Because they are relatively rare in the vicinity of the Sun, we know of few cool subdwarfs in the solar neighbourhood, and none for which both the mass and the radius are accurately determined. This hampers our understanding of stars at the low-mass end of the main sequence. Here we report the discovery of SDSSJ235524.29+044855.7 as an eclipsing binary containing a cool subdwarf star, with a white dwarf companion. From the light curve and the radial-velocity curve of the binary we determine the mass and the radius of the cool subdwarf and we derive its effective temperature and luminosity by analysing its spectral energy distribution. Our results validate the theoretical relations between mass, radius, effective temperature and luminosity for low-mass, low-metallicity stars.

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Data availability

This work was based on observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the island of La Palma (program GTC21-18B), and on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 2100.D-5022(A). This publication makes use of VOSA, developed under the Spanish Virtual Observatory project supported from the Spanish MINECO through grant AyA2017-84089. This work made use of data from the European Space Agency (ESA) mission Gaia (, processed by the Gaia Data Processing and Analysis Consortium (DPAC, Figures 1, 3 and 4 and Supplementary Figs. 1, 2 and 4 have associated raw data. The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Code availability

The spectral/decomposition routine and the radial-velocity fitting method used in this work are not publicly available. The light-curve fitting method is available at MIDAS is available at VOSA is available at iSpec is available at The binary_c stellar evolution code is available at The X-Shooter reduction pipeline (version 2.9.3) is available at and the dedicated HiPERCAM pipeline is available at

Additional information

Journal peer review information: Nature Astronomy thanks Sebastien Lepine and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.


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This work was partially supported by the MINECO Ramón y Cajal programme RYJ-2016-20254 (to A.R.M.) and grant AYA2017-86274-P (to A.R.M. and S.T.) and by the AGAUR grant SGR-661/2017 (to A.R.M. and S.T.). S.G.P. acknowledges the support of the Leverhulme Trust. J.J.R. acknowledges support from the Joint Funds of the National Natural Sciences Foundation of China (grants U1531244 and U1831209), the NSFC grant 11833006 and the Young Researcher Grant of the National Astronomical Observatories, Chinese Academy of Sciences. HiPERCAM and V.S.D. are funded by the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013) under ERC-2013-ADG grant agreement number 340040 (HiPERCAM). We thank F. Jimémez-Esteban for advice on the use of VOSA, A. Kesseli for sharing her ultra-subdwarf spectra and L. Althaus for discussions.

Author information

All authors contributed to the work presented in this paper. A.R.M. performed the decomposition and fitting of the spectra, carried out the entire spectral analysis (except the one required by iSpec), conducted the VOSA analysis and led the writing of the manuscript. S.G.P. reduced all the spectroscopic and photometric data and carried out the light-curve analysis. V.S.D. and S.P.L. performed the GTC observations. J.J.R. conducted the iSpec analysis. V.S.D., S.P.L. and T.R.M. contributed to the development of HiPERCAM, a vital instrument for obtaining the results of this work. S.T. carried out the binary_c simulation and calculated the cooling age of the white dwarf. A.R.M., S.G.P. and J.J.R. discovered the system. All authors reviewed the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to Alberto Rebassa-Mansergas.

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  1. Supplementary Information

    Supplementary Figs. 1–4, Supplementary Table 1.

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Fig. 1: X-Shooter optical spectrum of SDSS J2355+0448.
Fig. 2: Periodogram obtained from the radial-velocity data of the cool subdwarf in the binary SDSS J2355+0448, derived from X-Shooter spectroscopy.
Fig. 3: Phase-folded HiPERCAM light curves of SDSS J2355+0448.
Fig. 4: The observational SED of the cool subdwarf in SDSS J2355+0448.
Fig. 5: The observed stellar parameter relations of the cool subdwarf in SDSS J2355+0448.