In this thesis, we have investigated the effects of the radiatively-driven microscopic diffusion of iron, carbon, nitrogen and oxygen in a typical $eta$~Cephei star. We thought that it was possible that microscopic diffusion could explain recent puzzling observations in some $eta$~Cephei stars, such as a wide range of observed frequencies ($ u$~Eri and 12~Lac), the existence of low metallicity $eta$~Cephei stars (observed in the SMC and the LMC), as well as hybrid $eta$~Cephei-SPB stars ($gamma$~Peg, $psi$~Cen), and unexplained carbon, nitrogen and oxygen abundance ratios ($delta$~Cet, $eta$~Cep, $xi^1$~CMa, V2052~Oph and to a lesser extent $ u$~Eri). In order to tackle the role of radiative forces and microscopic diffusion in $eta$~Cephei stars, we had to implement them in our stellar evolution code. In this process, we also had to add the effects of mass loss through stellar winds in order to remove surface abundance anomalies and numerical instabilities. We have shown that the radiative forces are able to sustain iron against gravity in $eta$~Cephei stars, that radiatively-driven microscopic diffusion is important in the external layers of $eta$~Cephei stars, and that it induces the accumulation of a significant amount of iron in the driving region of the pulsation modes, which is the iron convective zone at 200,000~K. This accumulation leads to an enhancement of the opacity and thus favors the $kappa$-mechanism responsible for the excitation of the pulsation modes. We have shown through parametric studies that indeed more modes become unstable. Our latest computations, involving a full evolutionary study, confirm the results of our parametric studies. This provides an explanation for the wide range of frequencies observed in some $eta$~Cephei stars. It can also explain the existence of the hybrid $eta$~Cephei-SPB pulsators, because the accumulation of iron broadens the instability strips for both the $eta$~Cephei and SPB stars. The exsitence of low metallicity $eta$~Cephei stars is also explained since microscopic diffusion can locally increase the iron in the driving region, creating at least a few unstable modes. Another important result from our work is that microscopic diffusion happens very early in the evolution of $eta$~Cephei stars, in fact as soon as the star is born. It would be interesting to check if the same is true for less massive stars, as it is usually assumed that they are homogeneous during the pre-main sequence. Our results for carbon, nitrogen and oxygen show that radiative forces could possibly explain the observed excess of nitrogen. They could offer a reasonable alternative to the usual argument of rotational mixing.

The theory of stellar evolution and stellar structure relies on the observation of stars in di erent phases of their evolutionary cycle. The relation between observations and theory can be strengthened by obtaining observational data of a large sample of stars in a particular evolutionary phase. The search for Cephei stars, as conducted in this study, can contribute to the sample of known Cephei stars, where these interesting stars are massive non-supergiant early B-type stars, displaying pulsating behaviour which is not well understood. Stars tend to form in clusters where it can therefore be expected that young massive stars can be found in open clusters. For this reason two young southern open clusters were observed in order to search for B-type pulsating stars. The region of NGC 6204 and Hogg 22 was observed over a period of thirteen nights in Johnson B, V and I bands. NGC 6204 is believed to be the oldest cluster of the two at a distance of 0.8 kpc while the much younger Hogg 22 is more distant at 2.8 kpc. These two open clusters are located 6 arcminutes apart which made it possible to observe them simultaneously with a 12.8 12.8 arcminute eld of view. The observations were done with the newly installed 1600 telescope of the North-West University, South Africa. In order to do a variability search, periodic stars need to be identi ed from the cluster data, where a typical data set may contain thousands of scienti c images. In addition to the main motivation for this study, a pipeline was created in order to automate the photometry and data reduction processes. A Lomb-Scargle transform was applied to the stellar light curves in order to identify periodic sources. 354 signi cantly periodic stars were identi ed from the 3182 observed stars. Amongst them, two new possible Cephei stars were found together with a possible slowly pulsating B star (SPB), and numerous eclipsing binary systems. By using photometry of this region obtained by Forbes & Short (1996), instrumental magnitudes were transformed to a standard system in order to compare photometry results. From the constructed colour magnitude diagram of the two clusters, it could be seen that some stars, indicated by Forbes & Short (1996) to be cluster members, were in fact eld stars belonging to neither cluster. The reduction and photometry pipeline was implemented successfully on the data set, which also highlighted the importance of instrumentation and correct data analysis procedures. Possible improvements were identi ed in order to overcome di culties experienced during this study. / Thesis (MSc (Space Physics))--North-West University, Potchefstroom Campus, 2013

The theory of stellar evolution and stellar structure relies on the observation of stars in di erent phases of their evolutionary cycle. The relation between observations and theory can be strengthened by obtaining observational data of a large sample of stars in a particular evolutionary phase. The search for Cephei stars, as conducted in this study, can contribute to the sample of known Cephei stars, where these interesting stars are massive non-supergiant early B-type stars, displaying pulsating behaviour which is not well understood. Stars tend to form in clusters where it can therefore be expected that young massive stars can be found in open clusters. For this reason two young southern open clusters were observed in order to search for B-type pulsating stars. The region of NGC 6204 and Hogg 22 was observed over a period of thirteen nights in Johnson B, V and I bands. NGC 6204 is believed to be the oldest cluster of the two at a distance of 0.8 kpc while the much younger Hogg 22 is more distant at 2.8 kpc. These two open clusters are located 6 arcminutes apart which made it possible to observe them simultaneously with a 12.8 12.8 arcminute eld of view. The observations were done with the newly installed 1600 telescope of the North-West University, South Africa. In order to do a variability search, periodic stars need to be identi ed from the cluster data, where a typical data set may contain thousands of scienti c images. In addition to the main motivation for this study, a pipeline was created in order to automate the photometry and data reduction processes. A Lomb-Scargle transform was applied to the stellar light curves in order to identify periodic sources. 354 signi cantly periodic stars were identi ed from the 3182 observed stars. Amongst them, two new possible Cephei stars were found together with a possible slowly pulsating B star (SPB), and numerous eclipsing binary systems. By using photometry of this region obtained by Forbes & Short (1996), instrumental magnitudes were transformed to a standard system in order to compare photometry results. From the constructed colour magnitude diagram of the two clusters, it could be seen that some stars, indicated by Forbes & Short (1996) to be cluster members, were in fact eld stars belonging to neither cluster. The reduction and photometry pipeline was implemented successfully on the data set, which also highlighted the importance of instrumentation and correct data analysis procedures. Possible improvements were identi ed in order to overcome di culties experienced during this study. / Thesis (MSc (Space Physics))--North-West University, Potchefstroom Campus, 2013