4 Conclusions

About $50\%$ of the observed stars (14 out of 28) in NGC 6611 present indications of measured intrinsic polarization. A similar situation was found in the polarimetric study of IC 2944 (Vega et al. [1994]), another open cluster in close relation with an H II region (RWC62). On the other hand, practically no observed members showed polarization due to a non-interstellar origin in Hogg 15 and Lynga 14 (Orsatti et al. [1998]), two open clusters but without any association with gas.

The NGC 6611 intracluster polarization amounts to $\overline {P_{ V}}$ = 1.99% (mean of 14 stars). When compared to the polarization found for the "frontside'' stars (1.88%), we confirm the presence of polarizing dust in association with the observed members of NGC 6611.

The mean intra-cluster $\lambda _{\rm max}$ value associated with the observed stars gives $\overline {\lambda_{\rm max}} = 0.62\pm 0.10 ~ \rm\mu m$(mean of 14 stars). This dispersion indicates the coexistence of different grain size distributions. When compared to the $\lambda _{\rm max}$ value for the general interstellar medium, it implies a certain enlargement in mean silicate grain size. Also, it was found that the canonical relation $R_{ V} = 5.5\lambda_{\rm max}$is not valid for stars in dusty H II regions like the observed portion of M 16 and the Carina Nebula where it was previously shown by Tapia et al. ([1988b]) and subsequently confirmed by Marraco et al. ([1993]). This may arise mainly from the presence of these before mentioned silicate grains of slightly larger size than the standard ISM and also from a considerable increase in mean graphite grain size, in accordance with previous results from Chini & Wargau ([1990]).

We plan to extend the observations to other parts of the H II region less connected with dense dust clouds to help clarify the validity of this non canonical relation and to re-observe for variability some stars of the present paper in order to further explain the circumstellar polarization in more detail.

Acknowledgements

We wish to acknowledge the technical support and hospitality at CASLEO during the observing runs and also the support of the Facultad de Ciencias Astronómicas y Geofísicas de La Plata (UNLP). Special thanks go to Mrs. M. C. Fanjul de Correbo and to Mr. Ruben E. Martínez for technical assistance. An anonymous referee, who pointed out several key items and generously improved the manuscript, is also thanked.