Confidence regions at the M–R plane for SAX J1810.8−2609 obtained from the χ² map using the cooling tail method modified for rapid rotation. The solid, dashed, and dotted curves indicate the 68%, 90%, and 99% regions, respectively. The red contours correspond to the inclination i = 60°, while the blue contours are for the face-on NS. The black contours give the constraints assuming a non-rotating NS. (a) CRF case, i.e. constant relative flux. (b) CAF case, i.e. constant absolute flux. The triple dot-dashed curves are the contours of the constant distance of 4.0 and 4.5 kpc (distance is the only free parameter for a given pair of M and R).