Variation of the observed X-ray fluxes derived with XSPEC for the ROSAT, Chandra, and XMM-Newton observations in the energy band 0.5−2 keV. ¹ROSAT observation 2 446 928.585, ²ROSAT observation 2 449 508.604. The phase 0 has been arbitrary chosen to the oldest ROSAT observation. We did not use the T₀ derived by Annuk (1990) because they strongly depend on the line considered. The period has been fixed to P = 1521.1 d. Error bars are related to the relative error of the count rate (see text).