1 Introduction

The open cluster NGC 6611 located near the external border of the Sagittarius-Carina spiral arm $(l, b) = 17.0^{\circ}, +0.8^{\circ}$has been a subject of many studies based on astrometric and astrophysical methods. This interest has been caused by the extremely young age of NGC 6611 and by the importance of such investigations for the understanding of star formation and evolution. The cluster is probably associated with the extensive bright emission nebula M 16 (The Eagle Nebula). Morgan et al. (1953) assumed NGC 6611 to be physically connected with the association Ser I.

The first extensive photometric study of NGC 6611 was carried out by Walker (1961) from photoelectric and photographic UBV observations of 532 stars down to V = $16.7^{\rm m}$. He used color excesses of stars as a criterion to pick out cluster members. Hoag et al. (1961) obtained photoelectric UBV photometry of 24 stars down to V = $14.7^{\rm m}$ in this region. The spectral classification and photoelectric UBV photometry for the 15 brightest stars was carried out by Hiltner & Morgan (1969). Sagar & Joshi (1979) determined photoelectric UBV magnitudes for 50 cluster members. The measurements of 89 stars in UBVRI photometric bands were carried out by Thé et al. (1990). Hillenbrand et al. (1993) studied this cluster by combining CCD observations in the UBV system of 1022 stars down to V = $18.7^{\rm m}$ with spectroscopic and infrared (JHK) photometric data. The CCD observations were also used to derive equatorial coordinates of these stars.

Using plates taken with the 30-cm double refractor of the Observatorium Hoher List, Van Schewick (1962) and Tucholke et al. (1986) determined proper motions of 231 stars down to B = $13^{\rm m}$ with a baseline of 40 years and of 253 stars down to B = $15.1^{\rm m}$ with a baseline of 64 years, respectively. Kamp (1974) obtained proper motions for 142 stars down to V = $13.2^{\rm m}$ with plates taken over 47 years with the Yerkes 40-inch refractor. In all these studies, proper motion membership probabilities were determined by assuming two different distribution functions for field and cluster stars.

The brightest stars in this region are included in the PPM-South catalogue (Bastian & Röser 1993). For about 200 stars, proper motions were derived by Röser (1996) from the comparison of positions from the Astrographic Catalogue and the Guide Star Catalog.

A study of the cluster NGC 6611 based on Schmidt plate observations is given by Kharchenko and Schilbach (1995), hereafter referred to as KS95. Equatorial coordinates, proper motions and photographic B, V magnitudes for about 36000 stars were derived in a field of 8.95 sq.degrees from three plates of the Tautenburg Schmidt telescope (134/203/401) with a scale of 51.4 arcsec/mm and a time-span of 24.2 years. The cluster membership probabilities were obtained in the four-dimensional phase space with two components for proper motions and two components for plate coordinates.

On the basis of all these data we compiled an astrometric and photometric survey down to V = $16.8^{\rm m}$ within a circular area of 0.45 sq.degrees centered at the cluster NGC 6611. With 2185 entries, the catalogue gives the most representative list of stars in this region and contains equatorial coordinates for the equinox 2000.0 and epoch 1990.77, proper motions in the Hipparcos system as well as optical and infrared photometric data. The catalogue is described in Sect. 2.

Using color excess data gathered in the catalogue, we analyse in Sect. 3 the spatial distribution of the absorption within the cluster, derive the reddening map of this region, construct the reddening-free color-magnitude diagrams, and determine the cluster distance.