Spectral classification of faint stars and its application to the study of open clusters, star forming regions and the galaxy. I. Spectral types and luminosities of stars to 22.0m(V) in the stellar clusters NGC 2244, NGC 2264, and NGC 6913 - Kuznetsov, V.; Buscombe, W.; Butenko, G.; Lazorenko, G.; Lazorenko, P. (Кинематика и физика небесных тел, 2003)