1. Alissandrakis E.C., Dialetis D. and Tsiropoula G. (1987), AandA 174, 275.
2. Atroshchenko I.N. and Gadun A.S. (1994), AandA 291, 635.
3. Bendlin C. and Volkmer R. (1993), ASIA 278, 601.
4. Berger T.E. and Title A.M. (1996), ApJ 463, 365.
5. Berger T.E., Löfdahl M.G., Shine R.S. and Title A.M. (1998), ApJ, in press.