1. Bevington, P. R.: 1969, in Data Reduction and Error Analysis for the Physical Sciences, McGraw-Hill Inc., New York, p. 208.
2. Blanchet, L., Cornille, M., Dubau, J., Faucher, P., Lion, J., and Volonté, S.: 1985, Astron, Astrophys. 152, 417.
3. Burek, A.: 1976, Space Sci. Inst. 2, 53.
4. Doschek, G. A., Kreplin, R. W., and Feldman, U.: 1979, Astrophys. J. 233, L157.
5. Doschek, G. A., Feldman, U., Landecker, P. B., and McKenzie, D. L.: 1981, Astrophys. J. 249, 372.