The peculiar features in the spectral flux distribution of CP2 stars cause deviations from the values of photometric indices of normal stars. As previous authors have already mentioned, the blanketing is increased in CP2 stars, so the m1 index is larger than for normal stars (Strömgren 1963, 1967; Cameron 1967) and the hottest stars do not satisfy the [u-b] vs. m1 relation although they satisfy the [u-b] vs. relation (Strömgren 1966). On the other hand, the Balmer jump decreases, so c1 decreases as well (Preston 1975; Gerbaldi et al. 1974). The hottest CP2 stars look bluer than normal stars and the coolest are too red (Wildey et al. 1962). The index also decreases (Hauck 1975). Henry (1969) showed that the peculiar A0-A3 stars are below the relation for normal stars in the Strömgren (b-y) vs. a diagram.
Most of the authors used m1 vs. c1 or m1 vs. (b-y) diagrams (Cameron 1967; Maitzen 1976; Hauck 1975; Adelman et al. 1995, among others) to separate peculiar from normal stars. However, since all Strömgren-Crawford indices are altered by the peculiarities, we make use of the whole photometric information concerning peculiarity for the sake of the discrimination.
The method used was the Multiple Discriminant Analysis also called the Canonical Discriminant Analysis (hereinafter MDA; see for example Murtagh & Heck 1985). This method analyzes g populations, formed by ng individuals, described by q variables. The g populations are represented along canonical orthogonal axes maximizing the spread of the means of the populations and restraining their compactness. The MDA is invariant under linear transformations of the variables and it takes into account the correlation between them. In our case, there are two populations (the normal stars and the peculiar stars) and the observed variables are the Strömgren-Crawford photometric indices. One axis is enough to represent two populations. We denote p the coordinate associated with this axis.