Identification of a complete sample of northern ROSAT All-Sky Survey X-ray sources: V. Discovery of a z=4.28 QSO near the RASS source RX J1028.6-0844 - Zickgraf, Franz-Josef; Krautter, Joachim; Thiering, Ingeborg; Appenzeller, Immo (Astronomy & astrophysics, 1997)