1. Gemini Observatory, Southern Operations Center, c/o AURA, Casilla 603, La Serena, Chile
2. Departamento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago, Chile
3. Observatorio Astronómico, Universidad Nacional de Córdoba, Córdoba, Argentina
4. Department of Astronomy, University of Maryland, College Park, MD 20742, USA