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We continue the study of O-supergiants belonging to the association Cyg OB2using moderate-resolution spectra. In this paper we present results of themodeling of the stellar atmosphere of Cyg OB2 #11. This object belongs to thespectral class Ofc, which was recently introduced and yet small in numbers. Ofcclass consists of stars with normal spectra with CIII{\lambda}{\lambda}4647,4650,4652 emission lines of comparable intensity tothose of the Of-defining lines NIII {\lambda}{\lambda}4634,4640,4642. Wecombined new spectral data obtained by the 1.5-m Russian-Turkish telescope withspectra from MAST and CASU archives and determined physical parameters of thewind and chemical composition of the stellar atmosphere using CMFGEN code. Theestimated nitrogen abundance is lower than one in atmospheres of normalO-supergiants (i.e. O4-6 supergiants without additional spectral index "n" or"c") and carbon abundance is solar. Also we find an excess in silicon. Wepresent an illustrative comparison of our modeling results with current Genevaevolutionary models for rotating massive stars. The position on theHertzsprung-Russell diagram corresponds to the star mass of about 50M\sun andage about 4.5 Myr. Moreover, we carried out the high angular resolution (about0.02'') observations on the Russian 6-meter telescope aiming to find weakercompanions of this star, which did not reveal any.

Investigation of CygOB2 #11 (O5 Ifc) by modeling its atmosphere