Radius and mass of a transiting M dwarf near the hydrogen-burning limit

OGLE-TR-123

¹ Observatoire de Genève, 51 Chemin des Maillettes,1290 Sauverny, Switerland e-mail: frederic.pont@obs.unige.ch
² LAM, Traverse du Siphon, BP 8, Les Trois Lucs, 13376 Marseille Cedex 12, France
³ Observatoire de Haute-Provence, 04870 St-Michel l'Observatoire, France
⁴ Centro de Astronomia e Astrofísica da Universidade de Lisboa, 1349-018 Lisboa, Portugal
⁵ European Southern Observatory, Casilla 19001, Santiago 19, Chile

Received: 24 June 2005
Accepted: 5 October 2005

Abstract

The system OGLE-TR-123, detected by the OGLE planetary transit survey in Carina, was shown by subsequent spectroscopic follow-up to be an eclipsing binary with a very low mass companion ( ). Neither the light curve nor the radial velocity data were sufficient to constrain the parameters more precisely and to exclude alternative explanations in terms of a triple system. In this paper, new HARPS radial velocity data and a precise photometric monitoring of the transit with VLT/FORS2 are presented, resulting in an improved mass and radius determination for OGLE-TR-123b and the rejection of alternative scenarios. We find  and  for the transiting companion, confirming its nature as a small M dwarf near the hydrogen-burning limit. Its mass-radius relation corresponds to expectations for a relatively young object. The primary is an F dwarf that does not rotate synchronously with the orbital motion. Asynchronous rotation is unexpected in such a close binary ( days) but could be explained by the small mass of the secondary, the young age for the system, and the absence of convective envelope in the primary. OGLE-TR-123b is one of only two known objects below 0.1  with directly determined radii, after a similar object was characterised around OGLE-TR-122. The characterisation of OGLE-TR-123 provides another example of the type of system that can closely mimic planetary transits and contaminate transit surveys.