Left and middle: evolution of the hardness ratios (HR = H/S = F^ISMcor(2.−10.0 keV) /F^ISMcor(0.5−2. keV)) as a function of observed fluxes. Red triangles correspond to the 2003 XMM-Newton data (the year with the largest number of observations), green crosses to the new data, and black open circles to the other XMM-Newton observations. The values shown in the left (resp. middle) panel correspond to the results from 4T fits (resp. 2T fits). Right: evolution of the observed fluxes from 2T fits as a function of phase (with 1 /P = 0.01838 d^-1 and T₀ = 2 451 751.707), along with the best-fit sinusoid (dotted line).