1 Introduction

The object MWC 342 was discovered as an emission line object by Merrill & Burwell (1933). On the basis of the spectrum, the star was classified Be and this classification is retained also by Stock et al. (1960) and Perraud (1961). A description of the spectrum was published by Swings & Struve (1943), who mention double emission components in the Balmer series, with narrow absorption cores characteristic of shells and numerous emission lines of Fe II and [Fe II]. Andrillat & Swings (1975) provided a description of the near infrared spectrum and Brosch et al. (1978) gave a description of the spectrum in 1974-75 based upon image tube spectra obtained at 75 and 223 Å/mm. Allen & Swings (1976) show a spectrum of the star and point out an important infrared excess.

The near infrared spectrum from IRAS observations was classified by Volk & Cohen (1989) as being of type "U''= unusual. The nature of most of these sources is unknown.

Polarization was investigated by Zickgraf & Schulte-Ladbeck (1989) who found intrinsic polarization and by Yudin (1994) who confirmed the intrinsic polarization and found it to be variable with a period of half the photometric period (66 days). The detailed behavior could be explained by the presence of a compact X-ray source in a binary system. Using data from Amnuel et al. (1979) he found an X-ray source whose position coincides within the error box with MWC 342.

Bergner et al. (1990) made a photometric study based upon observations in the UBVRIJHK bands, whose conclusion can be summarized as follows. A model is proposed in which the star has a temperature of 28 000 K, whereas the temperatures of gas and dust are respectively 9000 K and 800 K. The spectral type of the star should correspond to B0-B0.5 on the ZAMS. With a visual absorption of 2.1 magnitudes, a distance of 1.1 kpc is obtained. A long term photometric variability of 132 days is also found, which seems to be related to the variable polarization. In no band the variation over the period is larger than 0.65 magnitudes.

The aim of the present study as well as of others of this series is to provide a complete line identification based upon the largest possible wavelength region, as well as a description of the spectrum and a comparison with previous work.