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We present optical photometric and polarimetric observations of stars towards NGC 1931 with the aim to derive the cluster parameters such as distance, reddening, age and luminosity/mass function as well as to understand the dust properties and star formation in the region. The distance to the cluster is found to be 2.3±0.3 kpc and the reddening E(B − V ) in the region is found to be variable. The stellar density contours reveal two clustering in the region. The observations suggest differing reddening law within the cluster region. Polarization efficiency of the dust grains towards the direction of the cluster is found to be less than that for the general diffuse interstellar medium (ISM). The slope of the mass function (-0.98±0.22) in the southern region in the mass range 0.8 < M/M ⊙ < 9.8 is found to be shallower in comparison to that in the northern region (-1.26±0.23), which is comparable to the Salpeter value (-1.35). The K-band luminosity function (KLF) of the region is found to be comparable to the average value of slope (∼0.4) for young clusters obtained by Lada & Lada (2003), however, the slope of the KLF is steeper in the northern region as compared to the southern region. The region is probably ionized by two B2 main-sequence type stars. The mean age of the young stellar objects (YSOs) is found to be 2±1 Myr which suggests that the identified YSOs could be younger than the ionizing sources of the region. The morphology of the region, the distribution of the YSOs as well as ages of the YSOs and ionizing sources indicate a triggered star formation in the region.

The extinction law and the variation of colour excess with position, luminosity as well as spectral class in young open star clusters NGC 663, NGC869, NGC 884, NGC 1502, NGC 1893, NGC 2244, NGC 2264, NGC 6611, Tr 14, Tr 15,Tr 16, Coll 228, Tr 37 and Be 86 have been studied. The difference in the minimum and maximum values of E(B-V) of cluster members has been considered as a measure of the presence of non-uniform gas and dust inside the clusters. Its value ranges from 0.22 to 1.03 mag in clusters under study, which indicates that non-uniform extinction is present in all the clusters. It has been noticed for the first time in NGC 1502 and Tr 37. It is also found that the differential colour excess in open clusters, which may be due to the presence of gas and dust, decreases systematically with the age of clusters indicating that matter is used either in star formation or blown away by hot stars or both. There is no uniformity in the variation of E(B-V) with either position or spectral class or luminosity.Comment: 11 pages, 8 figures, 4 tables; accepted for publication in MNRAS, typos adde

Remarkable inconsistencies between elemental abundances in the main sequence stars and their progeny F-G supergiants are discussed. Comparative abundance analysis of the hot main sequence stars, the cepheid U Sgr and two cool supergiants belonging to young open cluster M 25 is performed. The detected disaccord in the abundances of carbon, oxygen and other elements between these stars having a common origin but occupying at present different evolutionary stages may be due to the fact that the chemical anomalies observed in B stars are caused by the mechanism of the radiative diffusion in the upper atmosphere layers. The chemical composition of B stars determined spectroscopically may not reflect correctly their true chemical composition, nor the chemical composition of the in-terstellar medium. On the other hand such abundance anomalies are not expected for F-G supergiants which have suffered the large scale mixing in the red giant phase. The observed abundances for these objects are much more reliable as a reference point in the study of galactic chemical evolution. Three new Be stars are discovered in M 25. Our study has doubled the number of Be stars known in this cluster.

Deep optical photometric data on the NGC 7538 region were collected and combined with archival data sets from Chandra, 2MASS and Spitzer surveys in order to generate a new catalog of young stellar objects (YSOs) including those not showing IR excess emission. This new catalog is complete down to 0.8 M ⊙ . The nature of the YSOs associated with the NGC 7538 region and their spatial distribution are used to study the star formation process and the resultant mass function (MF) in the region. Out of the 419 YSOs, ∼91% have ages between 0.1 to 2.5 Myr and ∼86% have masses between 0.5 to 3.5 M ⊙ , as derived by spectral energy distribution fitting analysis. Around 24%, 62% and 2% of these YSOs are classified to be the Class I, Class II and Class III sources, respectively. The X-ray activity in the Class I, Class II and Class III objects is not significantly different from each other. This result implies that the enhanced X-ray surface flux due to the increase in the rotation rate may be compensated by the decrease in the stellar surface area during the pre-main sequence evolution. Our analysis shows that the O3V type high mass star 'IRS 6' might have triggered the formation of young low mass stars up to a radial distance of 3 pc. The MF shows a turn-off at around 1.5 M ⊙ and the value of its slope 'Γ' in the mass range 1.5