Context : It appears that most ( if not all ) massive stars are born in multiple systems .
At the same time , the most massive binaries are hard to find due to their low numbers throughout the Galaxy and the implied large distances and extinctions .

Aims : We want to study : [ a ] LS III +46 11 , identified in this paper as a very massive binary ; [ b ] another nearby massive system , LS III +46 12 ; and [ c ] the surrounding stellar cluster , Berkeley 90 .

Methods : Most of the data used in this paper are multi-epoch high-S/N optical spectra though we also use Lucky Imaging and archival photometry .
The spectra are reduced with devoted pipelines and processed with our own software , such as a spectroscopic-orbit code , CHORIZOS , and MGB .

Results : LS III +46 11 is identified as a new very-early-O-type spectroscopic binary [ O3.5 If* + O3.5 If* ] and LS III +46 12 as another early O-type system [ O4.5 V ( ( f ) ) ] .
We measure a 97.2-day period for LS III +46 11 and derive minimum masses of 38.80 \pm 0.83 M _ { \odot } and 35.60 \pm 0.77 M _ { \odot } for its two stars .
We measure the extinction to both stars , estimate the distance , search for optical companions , and study the surrounding cluster .
In doing so , a variable extinction is found as well as discrepant results for the distance .
We discuss possible explanations and suggest that LS III +46 12 may be a hidden binary system , where the companion is currently undetected .

Conclusions :