Observation of periodic variable stars towards the Galactic spiral arms by EROS II

We present the results of a massive variability search based on a photometric survey of a six square degree region along the Galactic plane at (l = 305, b = 0:8 )a nd (l = 330, b = 2:5). This survey was performed in the framework of the EROS II (Exp erience de Recherche d'Objets Sombres) microlensing program. The variable stars were found among 1 913 576 stars that were monitored between April and June 1998 in two passbands, with an average of 60 measurements. A new period-search technique is proposed which makes use of a statistical variable that characterizes the overall regularity of the flux versus phase diagram. This method is well suited when the photometric data are unevenly distributed in time, as is our case. 1362 objects whose luminosity varies were selected. Among them we identied 9 Cepheids, 19 RR Lyr, 34 Miras, 176 eclipsing binaries and 266 Semi-Regular stars. Most of them are newly identied objects. The cross-identication with known catalogues has been performed. The mean distance of the RR Lyr is estimated to be4:9 0:3 kpc undergoing an average absorption of3:4 0:2 mag. This distance is in good agreement with that of disc stars that contribute to the microlensing source star population. Our catalogue and light curves are available electronically from the CDS, Strasbourg and from our Web site.