Balmer lines in emission were first detected in the line spectrum of HD 50138 by Merrill & Humason (1921). The first detailed description of the spectrum was made by Merrill (1931) who suggested the existence of two cycles of variability - a longer one of about five years and a shorter one of about thirthy days. He also found [O I] in weak emission. Later one a number of studies were devoted to the study of the blue spectrum, such as those by Struve & Swings (1940) and Swings & Struve (1943), Merrill (1952), Doazan (1960), Houziaux (1961) and Jaschek et al. (1980). The near infrared spectrum was studied by Houziaux (1961), Andrillat & Houziaux (1967), Jaschek et al. (1992) and the ultraviolet by Sitko et al. (1981) and by Hutsemekers (1985).
Allen & Swings (1976) classified the star into their group I, i.e. almost conventional Be stars with slightly extended atmospheres. The object is not associated with nebulosity and the interstellar and circumstellar extinction is small (<0.10 m) From polarimetrc studies Vaidya et al. (1994, 1995) conclude that the star has a circumstellar disc. This is in line with its IRAS infrared excess (Hu Jingyao & Zhou Xu 1990).
Two recent outbursts of the star are documented: the first in 1978-9 (Hutsemekers) and the second in 1990-1 (Andrillat & Houziaux 1991; Bopp 1993).
Recently Pogodin (1997) has shown that a rapid
day-to-day spectral variability exists too,
through a study of the profiles of H
, He I 5876 and the Na I lines.
The photometry of the star shows non-periodic variations of delta 
,
delta 
, delta 
delta 
Kilkenny et al. 1985). Such ranges are confirmed
by Halbedel (1991) who found no extra variations during the shell episode.
Jaschek et al. (1992) pointed out that the spectral types derived from the different spectral regions do not agree. These authors conclude that the object is intermediate between Be and B[e]. Such a classification disagrees with that of other authors who call the object a Herbig Be/Ae star. We shall come back to this point toward the end of the paper.
The aim of the present paper, as well of the others of this series, is to provide an identification list over an extended wavelength interval, together with a list of equivalent widths of the emission lines. This should lead to an improved understanding of B[e] stars and related objects.