  The compact association Cygnus OB2 is known to contain a large population of
massive stars, but its total mass is currently a matter of debate. While recent
surveys have uncovered large numbers of OB stars in the area around Cyg OB2,
detailed study of the optically brightest among them suggests that most are not
part of the association. We observed an additional sample of optically faint OB
star candidates, with the aim of checking if more obscured candidates are
correspondingly more likely to be members of Cyg OB2. Low resolution spectra of
9 objects allow the rejection of one foreground star and the selection of four
O-type stars, which were later observed at higher resolution. In a subsequent
run, we observed three more stars in the classification region and three other
stars in the far red. We identify five (perhaps six) new evolved very massive
stars and three main sequence O-type stars, all of which are likely to be
members of Cyg OB2. The new findings allow a much better definition of the
upper HR diagram, suggesting an age ~2.5Myr for the association and hinting
that the O3-5 supergiants in the association are blue stragglers, either
younger or following a different evolutionary path from other cluster members.
Though the bulk of the early stars seems to belong to an (approximately)
single-age population, there is ample evidence for the presence of somewhat
older stars at the same distance. Our results suggest that, even though Cyg OB2
is unlikely to contain as many as 100 O-type stars, it is indeed substantially
more massive than was thought prior to recent infrared surveys.
