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Context: It appears that most (if not all) massive stars are born in multiplesystems. At the same time, the most massive binaries are hard to find due totheir low numbers throughout the Galaxy and the implied large distances andextinctions. AIMS: We want to study: [a] LS III +46 11, identified in thispaper as a very massive binary; [b] another nearby massive system, LS III +4612; and [c] the surrounding stellar cluster, Berkeley 90. Methods: Most of thedata used in this paper are multi-epoch high-S/N optical spectra though we alsouse Lucky Imaging and archival photometry. The spectra are reduced with devotedpipelines and processed with our own software, such as a spectroscopic-orbitcode, CHORIZOS, and MGB. Results: LS III +46 11 is identified as a newvery-early-O-type spectroscopic binary [O3.5 If* + O3.5 If*] and LS III +46 12as another early O-type system [O4.5 V((f))]. We measure a 97.2-day period forLS III +46 12 and derive minimum masses of 38.80$\pm$0.83 M_Sol and35.60$\pm$0.77 M_Sol for its two stars. We measure the extinction to bothstars, estimate the distance, search for optical companions, and study thesurrounding cluster. In doing so, a variable extinction is found as well asdiscrepant results for the distance. We discuss possible explanations andsuggest that LS III +46 12 may be a hidden binary system, where the companionis currently undetected.

The little-studied cluster Berkeley 90. I. LS III +46 11: a very massive O3.5 If* + O3.5 If* binary