Variable stars in the open cluster NGC 6811

We performed a time-series B, V band CCD photometric observations of the open cluster NGC 6811 to search for variable stars. A total number of 16 variables were detected including nine new ones. They are classified as 12 delta Scuti stars and four unknown type variables. The 12 delta Scuti variables are all very likely members of the cluster. This makes NGC 6811 a very important target for asteroseismology of dwarf Cepheids. In addition, we have carried out an isochrone fitting on the V vs. B-V CMD of the cluster based on theoretical models. It gives an age of log(t) = 8.76 +/- 0.09 for the cluster with the solar metallicity, (Z=0.019). The distance modulus and color excess of NCC 6811 are determined to be (m - M)(v) = 10.59 +/- 0.09 and E(B - V) = 0.12 +/- 0.05, respectively. A preliminary Fourier analysis shows a few very interesting sequences of period-luminosity relations, with the fundamental mode matches the P-L relation defined by previous studies for delta Scuti variables with high precision observations. This also confirms that all the 12 delta Scuti stars observed in this work are highly probable members of NGC 6811. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.