Based on observations collected at the European Southern Observatory, Chile. Program 073.D-0354(A).

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Appendix A is only available in electronic form at http://www.aanda.org

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...

Current address: Department of Physics and Astronomy, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK.

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... stars

When mentioning "background star'', we refer to any stellar object that does not belong to the system, including foreground stars.

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... secondaries

We use the term "secondary'' for any stellar component in the field near the target star. A secondary can be a companion star or a background star.

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... noise

Note that a peak flux of 2.5-3 times the background noise corresponds to a total flux with a much larger significance, since the flux of a faint companion is spread out over many pixels. All detected components in our survey have a signal-to-noise ratio larger than 12.8.

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... $0.5 \pm 0.5\%$

Note that we only find one brown dwarf companion with $12~{\rm mag} \leq K_{\rm S} \leq 14~{\rm mag}$ . We find no background stars in this region, so accidental misclassification of companions as background stars is not an issue here.

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... $K_{\rm S}=14$ mag

In this section we denote the observed quantities with a star as a subscript. For example, R denotes the substellar-to-stellar companion ratio (including all brown dwarfs), while $R_\star$ indicates the observed substellar-to-stellar companion ratio, including only the brown dwarfs brighter than $K_{\rm S}=14$ mag.

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