1 Introduction

In a recent paper Gray et al. (2001) give a precise spectral classification of 372 late-A to early-G stars and announce the discovery of a new $\lambda$ Boostar: HD 174005. The spectrum is described as that of a star with the temperature of an A7-type star but with the K line and metallic line strengths of an A2-type star.

The spectra of the A-F type stars classified as $\lambda$ Booare characterized by weak lines of most metallic species (with the possible exception of C, N, O and S); the interpretation of the phenomenon responsible for these anomalous atmospheric abundances is still controversial in spite of the large efforts made to define the spectral peculiarities common in these stars and to interpret the position of these stars in the HR diagram in terms of their evolutionary stage.

The non-homogeneous properties of $\lambda$ Boostars have been confirmed by the analyses extended to spectral ranges other than the classical optical region. For example the IRAS data made it possible to detect the presence of an IR excess, interpreted as the signature of the presence of circumstellar matter around some of these stars. A broad absorption feature centred at 1600 Å has been detected in some $\lambda$ Boostars by studying their IUE spectra; this feature, due to a Lyman ${\alpha}$ satellite, is present in white dwarfs and FHB stars too and its detectability in $\lambda$ Boostars is made possible, in the current opinion, by the lower opacity sources of $\lambda$ Boostars compared to "normal'' A-type stars. These are the main, but not the only, heterogeneous properties of this class of stars.

A detailed inspection of the global situation prompted us to investigate an alternative explanation of the so-called $\lambda$ Boophenomenon, that of the duplicity of these stars and we found several observational facts to support this new hypothesis as described in Faraggiana & Bonifacio (1999) and in Faraggiana et al. (2001). In fact a composite spectrum, not detected as such, can be easily confused with that of a single metal-deficient star. Several catalogues which provide $\lambda$ Booclassifications are in fact contaminated by unrecognized binaries and the abundance analyses of such objects, treated as single sources, are meaningless. The purpose of our study is to detect the binary stars considered up to now as single peculiar objects.

HD 174005 represents a good example to confirm our hypothesis. We discuss in the next sections the photometric and spectroscopic characteristics of this object.