The beat Cepheids in the Magellanic Clouds: an analysis from the EROS-2 database ^*,**

¹ Institut d'Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98 bis Boulevard Arago, 75014 Paris, France e-mail: marquett@iap.fr
² Physics Department, University of Florida, Gainesville, FL 32611-8440, USA
³ Konkoly Observatory, Budapest, PO Box 67, 1525, Hungary
⁴ Research School of Astronomy & Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston ACT 2611, Australia
⁵ CEA, DSM, DAPNIA, Centre d'Études de Saclay, 91191 Gif-sur-Yvette Cedex, France
⁶ Observatoire Midi-Pyrénées, Laboratoire d'Astrophysique (UMR 5572), 14 av. E. Belin, 31400 Toulouse, France
⁷ Centrum Astronomiczne im. M. Kopernika, Bartycka 18, 00-716 Warszawa, Poland
⁸ Obserwatorium Astronomiczne, Uniwersytet A. Mickiewicza, Sloneczna 36, 60-286 Poznan, Poland
⁹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁰ Laboratoire de l'Accélérateur Linéaire, IN2P3 CNRS, Université de Paris-Sud, 91405 Orsay Cedex, France
¹¹ The Niels Bohr Institute, Copenhagen University, Juliane Maries Vej 30, DK2100 Copenhagen, Denmark
¹² Department of Astronomy, Ohio State University, Columbus, OH 43210, USA
¹³ Division of Medical Imaging Physics, Johns Hopkins University, Baltimore, MD 21287-0859, USA
¹⁴ LPNHE, CNRS-IN2P3 and Universités Paris 6 & Paris 7, 4 place Jussieu, 75252 Paris Cedex 05, France
¹⁵ Observatoire Astronomique de Strasbourg, UMR 7550, 11 rue de l'Université, 67000 Strasbourg, France
¹⁶ Observatoire de Marseille, 2 Place Le Verrier, 13248 Marseille Cedex 04, France
¹⁷ European Southern Observatory (ESO), Casilla 19001, Santiago 19, Chile
¹⁸ San Pedro de Atacama Celestial Exploration, Casilla 21, San Pedro de Atacama, Chile

Received: 21 August 2008
Accepted: 13 November 2008

Abstract

Context. A number of microlensing dark-matter surveys have produced tens of millions of light curves of individual background stars. These data provide an unprecedented opportunity for systematic studies of whole classes of variable stars and their host galaxies.

Aims. We aim to use the EROS-2 survey of the Magellanic Clouds to detect and study the population of beat Cepheids (BCs) in both Clouds. BCs pulsating simultaneously in the first overtone and fundamental modes (FO/F) or in the second and first overtone modes (SO/FO) are of particular interest.

Methods. Using special software designed to search for periodic variables, we have scanned the EROS-2 data base for variables in the typical period range of Cepheids. Metallicities of FO/F objects were then calculated from linear nonadiabatic convective stellar models.

Results. We identify 74 FO/F BCs in the LMC and 41 in the SMC, and 173 and 129 SO/FO pulsators in the LMC and SMC, respectively; 185 of these stars are new discoveries. For nearly all the FO/F objects we determine minimum, mean, and maximum values of the metallicity.

Conclusions. The EROS data have expanded the samples of known BCs in the LMC by 31%, in the SMC by 110%. The FO/F objects provide independent measures of metallicities in these galaxies. The mean value of metallicity is 0.0045 in the LMC and 0.0018 in the SMC.