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CoRoT 101186644: A transiting low-mass dense M-dwarf on an eccentric 20.7-day period orbit around a late F-star
L. Tal-Or, T. Mazeh, R. Alonso, F. Bouchy, J. Cabrera, H. J. Deeg, M. Deleuil, S. Faigler, M. Fridlund, G. Hebrard, C. Moutou, A. Santerne, and B. Tingley


We present the study of the CoRoT transiting planet candidate CoRoT 101186644, also named LRc01\_E1\_4780. Analysis of the CoRoT lightcurve and the HARPS spectroscopic follow-up observations of this faint (m$_V=16$) candidate revealed an eclipsing binary composed of a late F-type primary (T$_{\rm eff}=6090\pm200$\,K) and a low-mass dense late M-dwarf secondary star on an eccentric ($e=0.4$) orbit with a period of $20.7$ days. The M-dwarf has a mass of $0.096\pm0.011$\,M$_{\odot}$, and a radius of $0.104_{-0.006}^{+0.026}$\,R$_{\odot}$, which makes it probably the smallest and most dense late M-dwarf reported so far. In addition, contrary to the well-known problem of inflated M-dwarfs, the radius of the secondary is probably consistent or might even be below the radius predicted by theoretical M-R models for M stars.
The high uncertainty of the secondary radius is due to the limited ability to place an upper limit on light contribution from a possible background star. Nevertheless, this discovery adds yet another piece to the puzzle of the mass-radius relations at the bottom of the main sequence.
We review the current agreement between observations and theory of very-low-mass stars (M\,$\lesssim0.2$\,M$_{\odot}$), and conclude that a large scatter of radii probably exists in this mass domain, but no clear discrepancy between models and observations can be deduced at this point.

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