Identification of a complete sample of northern ROSAT All-Sky Survey X-ray sources: II. the optical observations - Zickgraf, Franz-Josef; Thiering, Ingeborg; Krautter, Joachim; Appenzeller, Immo; Kneer, Reinhold; Pakull, Manfred W.; Heidt, Jochen (Astronomy & astrophysics. Supplement series, 1997)