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Astron. Astrophys. 340, 149-159 (1998)

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1. Introduction

Solar and Astero-seismology, as a new branch of science, has been, since its beginning in the early 1980's, an important tool to have a better knowledge of the stars themselves and of their interior in particular. Actual models are zeroth-order approximations and a careful selection of target stars, will give us, new and important information to improve the theoretical stellar models given the observational constraints.

Delta Scuti stars are a group of well studied variable stars and probably for the same reason, they are very good candidates for trying to understand the detailed phenomena that can be observed with new techniques. They have been carefully studied and multi-periodicity is a common feature on several of them. Many examples of this feature can be found in the literature. Belmonte et al. (1998) show a recent review of this multi-periodicity in Praesepe [FORMULA] Scuti stars. In field objects, FG Vir is the most relevant example with 24 detected modes (Breger et al. 1998).

The STEPHI program (Stellar Photometry International), (see for example Michel et al. 1995) is an international program where French, Spanish, Mexican and Chinese astronomers join their common scientific interests to improve the knowledge of the structure of [FORMULA] Scuti stars.

The importance of long, continuous ground based data sets, including multi-techniques, has proven its usefulness. STACC (Frandsen 1992, Frandsen et al. 1996), DSN, Breger et al. 1995; WET (Nather et al. 1990, Handler et al. 1997), MUSICOS (Be-Star Newsletter 1997), Be star observations network (Sareyan et al. 1998) and others, are some examples of well proven efforts of coordinated programs in longitude.

The first observations of the STEPHI program were obtained in 1987 and since that time, we have contributed to the research of the new branch of science known as Astero-seismology. In this work, we report the observational results of BQ and BW Cnc, two stars of Praesepe cluster observed during January-February 1995. BU and BN Cnc of the same cluster were observed in 1992 (Belmonte et al. 1994a) and two more, BS and BT Cnc in 1996 (Hernández et al. 1998). Other stars observed under the STEPHI program are the following: 63 Her; GX Peg; V650 Tau; VW Ari; and V647 Tau. See Belmonte et al.(1998) and references on that paper for a review of the program.

The information on the pulsation frequencies of [FORMULA] Scuti stars of a given cluster, will help us to provide some of the observational constraints necessary to have a better understanding of the stellar structure. The known parameters of the cluster, like age and chemical composition, will help us to better determine the theoretical characteristics of the stars. Large efforts in searching for new [FORMULA] Scuti stars in other northern-hemisphere clusters have been performed recently (Viskum et al. 1997).

The outline of this work is as follows. In Sect. 2, our target stars for this campaign and their characteristics are described. The acquisition of the data and the description of the observations will be shown in Sect. 3. The frequency analysis to get the set of detected modes is developed in Sect. 4 and the results of our work are given in Sect. 5. Finally, the conclusions will be shown in Sect. 6.

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© European Southern Observatory (ESO) 1998

Online publication: November 3, 1998