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Astron. Astrophys. 333, 524-530 (1998)

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2. Status of search

Observations of [FORMULA]  Scuti stars in the southern cluster NGC 6134 have been published by Frandsen et al. (1996). Further work is in progress on this cluster based on Strömgren photometry from May 1996. Spectroscopic data are still needed before a full model analysis can be carried out (e.g. no [FORMULA] exists for the pulsating stars).

Another southern cluster, NGC 2660, contains a number of [FORMULA]  Scuti stars, but is quite distant, [FORMULA] Kpc. At such a distance supplementary spectral observations can only be done using very large telescopes and long exposures.

After the two-site observations of NGC 6134, STACC activities have been devoted to the search for a similar northern cluster, for which results are presented by Viskum et al. (1997a). In the present paper we present the discovery of a second open cluster, NGC 1817, with a nice population of [FORMULA]  Scuti stars in a field of view small enough to observe all pulsators simultaneously.

Aware of the difficulty of locating clusters like NGC 6134, we have also worked on the nearby cluster Praesepe, which is known to have the largest number of pulsators of any open cluster. This cluster does not offer the opportunity to observe many pulsators simultaneously, which is a major disadvantage. Observations of individual stars in Praesepe are described by Breger et al. (1993; 1994) and Belmonte et al. (1994). There happens to be a pair of [FORMULA]  Scuti stars, BN Cnc and BV Cnc, which can be observed simultaneously. Observations by STACC members recently led to the discovery of 4 new frequencies in BV Cnc (Arentoft et al. 1997).

In view of recent developments in the field, our strategy has been modified somewhat. Our goal is now to obtain more extensive and complete data sets (Sect.  5).

In order to support groups that want to participate in the search for clusters with a large population of variables, we have compiled a document (`The Book'): The STACC Open Cluster Target List (Frandsen & Arentoft, 1998).

2.1. Content and purpose of The Book

The idea of STACC is that the programs can be carried out with CCD cameras on small ([FORMULA] m) telescopes. The observations are simple. They offer students or advanced amateurs an opportunity to learn how to do high precision CCD photometry. The observations have to be done carefully along lines that have been described by Gilliland & Brown (1988) and Kjeldsen & Frandsen (1992).

We have collected all the relevant information we could find on each cluster. The document contains an introduction to the subject with some guidelines for making the observations. Then follows the main section on a number of target clusters (mostly northern).

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998