We have investigated the two open clusters NGC 6192 and NGC 6451 for which widely different reddening values and thus ages and distances via Stroemgren uvby photometry can be found in the literature. Our measurements allow us to disentangle the apparent discrepancies in the literature and to derive new accurate values. From appropriate calibrations we find that the overall abundance for NGC 6192 is about solar whereas a subsolar value for NGC 6451 was estimated. From two previous reported photometrically candidate CP stars, one within NGC 6192 shows Stroemgren indices typical for a B8Si star whereas the other object of NGC 6451 is most probably a foreground G-type star.
CCD photometry on the intermediate-band uvbyCaH{beta} system is presented for the open cluster, NGC 3680. Restricting the data to probable cluster members using the color-magnitude diagram (CMD) and the photometric indices alone defines a sample of 34 stars at the cluster turnoff that imply E(b-y)=0.042+/-0.002(s.e.m.) or E(B-V)=0.058+/-0.003(s.e.m.), where the errors refer to internal errors alone. With this reddening, [Fe/H] is derived from both m_1_ and hk using both b-y and H{beta} as the temperature indices.
By studying young open clusters, the mechanisms important for star formation over several Myr can be examined. For example, accretion rate as a function of rotational velocity can be investigated. Similarly, sequential star formation triggered by massive stars with high mass-loss rates can be studied in detail. We identified and characterized probable members of NGC 6383, as well as determined cluster parameters. New Stromgren uvby CCD photometry, obtained by us, is presented. This new data, together with Johnson UBV and 2MASS data in the NIR, was used to investigate characteristics of pre- as well as zero age main sequence cluster members. We present Stromgren uvby CCD photometry for 272 stars in the field of NGC 6383 and derive its reddening, E(b-y)=0.21(4)mag, as well as distance, d=1.7(3)kpc from the Sun. Several stars with NIR excess and objects in the domain of the classical Herbig Ae/Be and T Tauri stars were detected. Two previously known variables were identified as rapidly-rotating PMS stars. The field population is clearly separated from the probable members in the color-magnitude diagram. NGC 6383 is a young open cluster, with an age of less than 4Myr, undergoing continuous star formation. True pre-main sequence members might be found down to absolute magnitudes of +6mag, with a variety of rotational velocities and stellar activities.
The tables give the results of the Stromgren photometry of the two southern open clusters NGC 6134 and NGC 3680, which were not printed in the paper in A&AS.
CCD photometry of the intermediate-age open cluster IC 4651 on the uvbyH{beta} system is presented and analyzed. By using a combination of the information from the color-magnitude diagram (CMD) and the color-color diagrams, a sample of 98 highly probable main-sequence cluster members with high photometric accuracy is isolated.
CCD photometry on the intermediate-band uvbyCaH system is presented for the old open cluster NGC 6253. Despite a high level of field star contamination because of its location toward the Galactic center, combination of the data from the multiple color indices with the core cluster sample derived from radial star counts leads to the identification of a set of highly probable, single cluster members.
CCD photometry on the intermediate-band vbyCaH{beta} system is presented for the metal-deficient open cluster NGC 2420. Restricting the data to probable single members of the cluster using the CMD and the photometric indices alone generates a sample of 106 stars at the cluster turnoff.
By collecting optical and infrared photometry and low-resolution spectroscopy, we have identified a large number of low-mass stars and brown dwarf candidates belonging to the young cluster (~5Myr) associated with the binary star {lambda} Orionis. The lowest mass object found is an M8.5 with an estimated mass of 0.02M_{Sun}_ (~0.01M_{Sun}_ for objects without spectroscopic confirmation). For those objects with spectroscopy, the measured strength of the H{alpha} emission line follows a distribution similar to other clusters with the same age range, with larger equivalent widths for cooler spectral types.