Letter to the Editor

A planet-sized transiting star around OGLE-TR-122^*

Accurate mass and radius near the hydrogen-burning limit

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

Received: 28 January 2005
Accepted: 12 February 2005

Abstract

We report the discovery and characterisation of OGLE-TR-122b, the smallest main-sequence star to date with a direct radius determination. OGLE-TR-122b transits around its solar-type primary every 7.3-days. With and , it is by far the smallest known eclipsing M-dwarf. The derived mass and radius for OGLE-TR-122b are in agreement with the theoretical expectations. OGLE-TR-122b is the first observational evidence that stars can indeed have radii comparable or even smaller than giant planets. In such cases, the photometric signal is exactly that of a transiting planet and the true nature of the companion can only be determined with high-resolution spectroscopy.