1 Introduction

Solutions to various problems of modern astrophysics and stellar astronomy necessitate knowing spectral characteristics of faint stars. In light of this, the use of multicolour photometric systems seems to be rather appropriate. One of the most informative systems is the Vilnius seven-colour system (Strayzis 1977) which permits us to carry out two-dimensional classification in a wide spectral range from O to M stars.

The problem of the present research consists of finding an efficient approach to spectral classification of O-M stars yet not leaving the framework of the standard three-colour UBV system. A simple opportunity to extend spectral studies to faintest stars is possible in this case and a large amount of information on UBV values of stars already stored in photometric databases can be used. Also important is the fact that observations in UBV system do not present particular difficulties.

Unfortunately the opportunities for application of the Q-method (Johnson 1958; Johnson & Morgan 1953), used for O-A0 star spectral classification on the basis of UBV photometry, are rather limited because of the ambiguity of this method. For this reason classification errors on temperature parameters can reach 2-3 classes (Kuznetsov 1986; Kuznetsov & Lazorenko 1992) whereas errors in the MK system (Johnson & Morgan 1953) or Abastumany system of spectral criteria (Kharadze & Bartaya 1960) are usually one order lower and do not exceed 1-2 subclasses.

Dependence of Q_UBV values on the intrinsic colour (B-V)₀ or spectrum of a star Sp has been studied by Strayzis (1977). It was shown that knowledge of the Q_UBV value does not allow us to determine the spectral class of a star or its luminosity. Thus some additional information is required in order to obtain a single-valued treatment of the star spectral characteristics.

In the present study as a main criterion which permits us to avoid multiplicity in the estimates of stellar spectral characteristics at the fixed value Q_UBV, we suggest using information on the character of absorbing dust matter distribution versus distance from the Sun. In practice it is more convenient to use dependence of colour excesses E(B-V) on the intrinsic distance module V₀ - M_V in some direction of the sky. It is easy to construct such dependence in any section of the Galaxy, in particular, in directions of young open clusters. At present extensive information on distribution of interstellar absorbing matter in the Galaxy is available, see for example Voroshylov & Khalandadze (1983).