Astron. Astrophys. 333, 918-925 (1998)

1. Introduction

RR Lyrae radial pulsators have been early recognized as interesting stellar objects, widely used as population tracers and distance indicators. In particular, the pulsational behavior of metal poor RR Lyrae in galactic globular clusters has stimulated a large amount of investigations both observational and theoretical. However, since the pioneering paper by Preston (1959), one knows that field RR Lyrae in the solar neighborhood have a metal rich component, not observed in galactic globular clusters, which appears characterized by objects with peculiarly short periods. The origin of such a behavior has been recently investigated by Bono et al. (1997a, BCCIM hereafter), who presented detailed theoretical predictions for the shape of lightcurves for different assumptions about star masses, chemical compositions, luminosities and temperatures. Accordingly, one can foresee the possibility of connecting several features of the lightcurves (such as the rising time and the occurrence of bumps or dips) to the structural parameters of the pulsating stars, testing the reliability of theoretical predictions and, possibly, deriving information on the evolutionary status of the observed objects.

However, one finds that several lightcurves available in the literature for metal rich field RR Lyrae rely on old photographic photometry, whose intrinsic inaccuracy does not allow a detailed analysis of the predicted features. Taking advantage of the CCD technology, one may now obtain rather accurate photometry even with small telescopes. In this paper we report the first results of such a program started at the Teramo Astronomical Observatory with the use of the 72 cm Teramo/Scuola Normale Telescope (TNT).

Fig. 1a-c. Identification maps for AW Dra, AQ Lyr and CN Lyr; comparison stars are also indicated

From the General Catalog of Variable Stars (4th Ed: Kholopov e al. 1988, GCVS4 hereafter) we selected the sample of RR Lyrae brighter than mag and with known periods which can be observed from Teramo and for which no accurate lightcurves are available in the literature. Among these objects, we chose as first targets the two metal rich ab-type RR Lyrae (RRab) AQ Lyr and CN Lyr. For AQ Lyr one has i.e. (Z=0.006) (Suntzeff et al. 1994), while for CN Lyr, Layden et al. (1996) give (Z=0.01). To our knowledge, for AQ Lyr one finds in the literature only seven photographic BV measurements (Sturch 1966), whereas CN Lyr has photoelectric, but rather scattered, UBV lightcurves by Oosterhoff (1962). To these two target stars we added the RRab AW Dra for which neither a lightcurve nor a metallicity estimate were encountered in the literature.

© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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