  We have searched for visual binaries with projected separations in the range
200-3000 AU (0.1"-1.5") among a sample of 96 stars in the massive young NGC
6611 cluster, 60 of them being subsequently identified as high probability
cluster members of mainly OB spectral type. This is the first visual binary
survey among such a large and homogeneous sample of high-mass stars. We find an
uncorrected binary frequency of 18+/-6% over the surveyed separation range.
Considering only binaries with mass ratios q>0.1, we find that OB stars in NGC
6611 host more companions than solar-type field stars. We derive mass ratios
for the detected binaries from their near-infrared flux ratios and conclude
that about half of the detected binaries have q<0.2, which does not contradict
the assumption that companion masses are randomly drawn from the initial mass
function. There is no evidence in our sample that wide-binary properties depend
upon the mass of the primary star. The high frequency of massive binaries in a
cluster as rich as NGC 6611 and the lack of a strong mass dependence of their
properties are difficult to reconcile with the scenario whereby massive stars
form as the result of mergers of smaller stars. The canonical protostellar
accretion scenario together with cloud fragmentation, on the other hand, can
naturally explain most of the observed binary properties, although the very
high stellar density in the protocluster is likely to require significant
modification to that picture as well.
