Fig. 3a and b. Observed column densities of SY Mus plotted against the reduced phase as defined in Vogel (1991) a . In b the reduced phases were corrected by 0.035. The open circles represent the points at phases 0.944, 0.930 and 0.917 and the filled circles the points at phases 0.091, 0.107, 0.119, 0.130, 0.161 and 0.176.