  Context: Be stars are massive dwarf or subgiant stars that present temporary
emission lines in their spectrum, and particularly in the Halpha line. The
mechanism triggering these Be episodes is currently unknown, but binarity could
play an important role. Aims: Previous observations with the VLT/VISIR
instrument (Kervella & Domiciano de Souza 2007) revealed a faint companion to
Achernar, the brightest Be star in the sky. The present observations are
intended to characterize the physical nature of this object. Methods: We
obtained near-IR images and an H-band spectrum of Achernar B using the VLT/NACO
adaptive optics systems. Results: Our images clearly show the displacement of
Achernar B over a portion of its orbit around Achernar A. Although these data
are insuficient to derive the orbital parameters, they indicate a period of the
order of 15 yr. The projected angular separation of the two objects in December
2007 was smaller than 0.15 arcsec, or 6.7 AU at the distance of Achernar.
Conclusions: From its flux distribution in the near- and thermal-IR, Achernar B
is most likely an A1V-A3V star. Its orbital period appears similar to the
observed pseudo-periodicity of the Be phenomenon of Achernar. This indicates
that an interaction between A and B at periastron could be the trigger of the
Be episodes.
