1 Introduction

NGC 6611 (C1816-138) is a stellar rich open cluster located in the Sagittarius spiral arm, in the plane of the Galaxy (l=17 $.\!\!^\circ$0, b =0 $.\!\!^\circ$8). It is embedded in an ionized hydrogen complex (M 16), an extensive nebulosity connected with dense dust clouds. The central portion of the cluster is part of the North-West of M 16, but nowadays it is assumed that it is spread all over the complex (e.g. de Winter et al. [1997]).

Prior to the era of the infrared observational campaigns, the extinction law in NGC 6611 was considered to be fairly normal. Gebel ([1968]) obtained a value of $R_{ V} = 2.5\pm0.6$ from a comparison of the radio emission measure for the H II region with H$\alpha$ and H$\beta$ fluxes. Johnson ([1968]) obtained a mean value of $R_{ V} = 2.9\pm0.5$ for NGC 6611 from an average of estimates from the variable-extinction technique ( $R_{ V} = 3.4\pm0.7$) and the cluster diameter method ( R _V = 2.3). The value obtained by Turner ([1994]) from the variable-extinction method was $R_{ V}=2.99\pm 0.08$, with only a localized region on the dusty north side of the cluster displaying evidence for a larger value of 4.42.

Many later investigations on NGC 6611, mostly based on IR studies (Sagar & Joshi [1979]; Chini & Krugel [1983]; Hillenbrand et al. [1993]; de Winter et al. [1997]; and others) concluded that the extinction in the NW area is caused either by intracluster or by circumstellar dust while showing variable degrees of anomaly. This in turn causes uncertainties in the determination of the distance to the cluster and consequently has some influence on the conclusions regarding the evolution of NGC 6611 itself. Also, it is generally accepted that the matter in front of the cluster is responsible for an excess E _B-V = 0.50 mag (e.g. Sagar & Joshi [1979]; Thé et al. [1990]; Hillenbrand et al. [1993]) and that a normal extinction law, characterized by $R_{ V} = 3.1\pm 0.1$, is valid for the foreground dust.

Our intention is to test independently the existence of abnormal extinction in NGC 6611 through the use of polarimetric techniques. Via polarimetry, it is possible to detect stars suspected of having circumstellar envelopes and to remove them from the rest; and then, to concentrate on the extinction caused by the intra-cluster dust.