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Astron. Astrophys. 351, L5-L9 (1999)

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1. Introduction

Two years ago, several very cold dwarfs were identified by DENIS (Delfosse et al. 1997) and Kelu 1 was found through its high proper motion (Ruiz et al. 1997). Follow-up observations immediately showed that their optical spectra bear little resemblance to those of the slightly hotter M dwarfs and resemble the previously atypical spectrum of GD 165B (Becklin & Zuckerman 1988; Kirkpatrick et al. 1993). Martín et al. (1997) suggested a new class for these objects, the L spectral class. Kirkpatrick et al. (1999) and Martín et al. (1999) take first steps towards a definition of this class. The main characteristic of L dwarf visible spectra, compared with those of M dwarfs, is the gradual disappearance of the VO and TiO molecular bands, now understood as due to depletion of titanium and vanadium into dust. This class contains both very low mass stars with masses just above the hydrogen burning limit and brown dwarfs, like DENIS-P J1228.2-1547 (Delfosse et al. 1997) and Kelu 1 (Ruiz et al. 1997).

High proper motions have historically been the first tool used to systematically search the solar neighbourhood for very low mass stars (Luyten 1925), and the discovery of Kelu 1 shows that this remains a powerful technique. In this letter we report the detection of two new L dwarfs in a proper motion survey using the EROS 2 instrument, and their confirmation by infrared photometry from DENIS. The EROS 2 proper motion survey primarily aims at halo white dwarfs, but a preliminary two-epochs analysis already has useful sensitivity to very cool disk objects. One of the new detections is a confirmed brown dwarf and the other is a borderline object, which may be either a star or a brown dwarf. The latter is a common proper motion companion of the parallax star LHS 102 (d=9.6 pc, M3.5V), and significantly improves the determination of the colour-luminosity relation for low luminosities L dwarfs. We first detail the observational setup and the selection process, and then discuss the two objects in some detail.

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© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999