\pm 0.2$ deg/$\mbox{yr}^2$, implying that the semimajor axis of the mutual orbit of the components decreases in time with a mean rate of $-2.8 \pm 0.2$ or $-5.1 \pm 0.2$ cm/yr for the two solutions, respectively.

    The expanding orbit of (66391) 1999 KW4 may be explained by mutual tides interplaying with binary YORP (BYORP) effect (McMahon, J., Scheeres, D. \[2010\]. Icarus 209, 494-–509). However, a modeling of the BYORP drift using radar-derived shapes of the binary components predicted a much higher value of the orbital drift than the observed one. It suggests that either the radar-derived shape model of the secondary is inadequate for computing the BYORP effect, or the present theory of BYORP overestimates it. It is possible that the BYORP coefficient has instead an opposite sign than predicted; in that case, the system may be moving into an equilibrium between the BYORP and the tides.

    In the case of (88710) 2001 SL9, the BYORP effect is the only known physical mechanism that can cause the inward drift of its mutual orbit.

    Together with the binary (175706) 1996 FG3 which has a mean anomaly drift consistent with zero, implying a stable equilibrium between the BYORP effect and mutual body 