(IAAC) Stars with spectral type F and G in NGC 6231

Last night I was observing the open cluster NGC 6231. Between its main
stars I could observe that some even showed to yellow color or orange.
In this Web, it appears a observacional report of William L. Schart,
in which also it says to observe stars of this spectral type.
Nevertheless, in several data bases, I have been able to state that
all the stars have spertral type O or B.  Somebody can clarify this
apparent contradiction to me.

Thank you very much,
Jorge Solera
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