A CCD search for variable stars of spectral type B in the Northern Hemisphere open clusters. III. NGC 6823

We present the results of variability search in the field of a very young o pen cluster NGC 6823. Two delta Sct stars, still at the pre-main sequence ( PMS) stage of evolution, were found. These objects could be used for future testing of the evolutionary period changes in this class of variable stars . In addition, we found 13 other variables including a bright cluster eclip sing binary and an SPB candidate. A few other variables could be PMS stars of the UX Ori type. The H alpha photometry, which we made for 69 brightest stars, revealed only one object with strong emission, discovered recently by the photographic m ethods. Two other stars announced to have H alpha emission, do not show any evidence for its presence. For a number of stars in the search field, we also provide the BV(RI)(C) ph otometry. We explain how the distribution of absorbing matter along the lin e of sight results in the unusual morphology of the cluster color-magnitude (CM) diagram. The dereddened CM diagram is used in the calculation of the extinction map for the observed field, in which the E(R - I)(C) color exces s varies from 0.54 to 0.72 mag, with the average value equal to 0.62 mag. It is also shown that all cluster stars with spectral types later than A0 a re PMS objects. We use these stars to estimate the cluster age: 3 +/- 1 Myr . Using the cluster CM diagram, we compare and discuss the position of the two discovered delta Sct stars with respect to the theoretical instability strip for PMS stars of this type.