CCD photometric search for peculiar stars in open clusters VIII. King 21, NGC 3293, NGC 5999, NGC 6802, NGC 6830, ruprecht 44, ruprecht 115, and ruprecht 120

Context. We continue our survey of magnetic chemically peculiar (CP2) stars in galactic open clusters to shed more light on their origin and evolution. Aims. To study the group of CP2 stars, it is essential to find these objects in different galactic environments and at a wide range of evolutionary stages. The knowledge of open cluster ages and metallicities can help for finding a correlation between these parameters and the (non-) presence of peculiarities, which has to be taken into account in stellar evolution models. Methods. The intermediate band \Delta a photometric system samples the depth of the 5200 Aa flux depression by comparing the flux at the centre with the adjacent regions with bandwidths of 110 Aa to 230 Aa. It is capable of detecting magnetic CP2 and CP4 stars with high efficiency, but also the groups of (metal-weak) \lambda Bootis and classical Be/shell stars can be successfully investigated. In addition, it allows the age, reddening, and distance modulus to be determined with appropriate accuracy by fitting isochrones. Results. From the 1677 observed members of the eight open clusters, one Ae and twenty-five CP2 stars were identified. Furthermore nineteen deviating stars are designated as questionable for several reasons. The estimated age, reddening, and distance for the programme clusters were compared with published values of the literature and discussed in this context. Conclusions. The current paper shows that CP2 stars are continuously present in very young (7 Myr) to intermediate age (500 Myr) open clusters at distances greater than 2 kpc from the Sun.