| Abstract
| - We investigate the multiplicity of three O-type stars in the very young open cluster IC 1805. All our targets were previously considered as spectroscopic binaries, but no orbital solution was available for any of them. Our results confirm the binarity of BD+60°497 and we provide the very first orbital solution for this double-lined spectroscopic binary. This is only the second O-star binary in IC 1805, and the first SB2 system, for which an orbital solution is now available. BD+60°497 has an orbital period of 3.96 days and consists of an evolved O6.5 V((f)) primary and an O8.5-9.5 V((f)) secondary with minimum masses of $m_1\,\sin^{3}{i} = 13.9~M_{odot}$ and $m_2\,\sin^{3}{i} = 10.9~M_{odot}$. The observed primary/secondary mass ratio (1.28) appears lower than expected from a comparison with single star evolutionary models (1.60-1.74). For the other two stars, BD+60°501 and BD+60°513, we find no significant radial velocity variations, suggesting that they are most probably single. Although a fraction of binaries among the early-type stars of IC 1805 as high as 80% has been advocated in the literature, our results suggest that this number might be overestimated.
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