Astronomy & Astrophysics manuscript no. DR2-binaries-1-v5r1 c ESO 2019 March 12, 2019 Multiplicity of Galactic Cepheids and RR Lyrae stars from Gaia DR2 I. Binarity from proper motion anomaly? Pierre Kervella1, Alexandre Gallenne2, Nancy Remage Evans3, Laszlo Szabados4, Frédéric Arenou5, Antoine Mérand6, Yann Proto7; 1, Paulina Karczmarek8, Nicolas Nardetto9, Wolfgang Gieren10, and Grzegorz Pietrzynski11 1 LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France, e-mail:
[email protected]. 2 European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago, Chile. 3 Smithsonian Astrophysical Observatory, MS 4, 60 Garden Street, Cambridge, MA 02138, USA. 4 Konkoly Observatory, MTA CSFK, Konkoly Thege M. út 15-17, H-1121, Hungary. 5 GEPI, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190 Meudon, France. 6 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany. 7 Ecole Normale Supérieure de Lyon, 15 parvis René Descartes, 69342 Lyon, France. 8 Warsaw University Observatory, Al. Ujazdowskie 4, 00-478, Warsaw, Poland. 9 Université Côte d’Azur, OCA, CNRS, Lagrange, France 10 Universidad de Concepción, Departamento de Astronomía, Casilla 160-C, Concepción, Chile. 11 Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, PL-00-716 Warszawa, Poland Received ; Accepted ABSTRACT Context. Classical Cepheids (CCs) and RR Lyrae stars (RRLs) are important classes of variable stars used as standard candles to estimate galactic and extragalactic distances. Their multiplicity is imperfectly known, particularly for RRLs. Astoundingly, to date only one RRL has convincingly been demonstrated to be a binary, TU UMa, out of tens of thousands of known RRLs.