Key points are not available for this paper at this time.
New photographic photometry is given for the open cluster NGC 6939. Using earlier proper motion measurements, a colour–magnitude diagram is obtained for those stars believed to be cluster members. The cluster appears to have an age of about 10 9 years, and has a low density region on the main sequence in agreement with a theoretically predicted gap. The red giant branch shows one very marked concentration which is tentatively identified with the horizontal branch found in globular clusters. There are a few anomalous bright blue stars, similar to those in other old clusters, well above the main sequence turn-off.
Cannon et al. (Sun,) studied this question.