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Context. Stellar physical properties of star clusters are poorly known. Aims.Our goals are to perform a spectrophotometric study of the B star population inopen clusters to derive accurate stellar parameters, search for the presence ofcircumstellar envelopes, and discuss the characteristics of these stars.Methods. The BCD spectrophotometric system is a powerful tool to obtainfundamental parameters and infer the main properties of open clusters: distancemodulus, color excess, and age. We inspected the Balmer discontinuity to seekcircumstellar disks and identify Be-star candidates. High resolution spectra inthe H$\alpha$ region are used to confirm the Be nature. Results. We provideTeff , log g, Mv , Mbol and spectral types for a sample of 68 stars in thefield of the open clusters NGC 6087, NGC 6250, NGC 6383, and NGC 6530, as wellas the cluster distances, ages and reddening. Then, based on a sample of 230 Bstars in the direction of the 11 open clusters studied along this series ofthree papers, we report 6 new Be stars, 4 blue straggler candidates, and 15B-type stars (called Bdd) with a double Balmer discontinuity. We also find thatthe majority of the Be stars are dwarfs and present a maximum at the spectraltype B2-B4 in young and intermediate-age open clusters. Another maximum of Bestars is observed at the spectral type B6-B8 in open clusters older than 40Myr, where the population of Bdd stars also becomes relevant. Conclusions. Ourresults support previous statements that the Be phenomenon is present along thewhole main sequence band and occurs in very different evolutionary states. Wefind clear evidence of an increase of stars with circumstellar envelopes withcluster age. The Be phenomenon reaches its maximum in clusters of intermediateage and the number of B stars with circumstellar envelopes (Be+Bdd stars) isalso high for the older clusters.

Open clusters: III. Fundamental parameters of B stars in NGC 6087, NGC 6250, NGC 6383 and NGC 6530. B type stars with circumstellar envelopes