Aims. The YORP eect is a small thermal-radiation torque considered to be a key factor in spin-state evolution of small Solar System bodies. In order to detect YORP on 1999 JV6 we developed a detailed shape model and analysed the spin-state using both optical and radar observations.
Methods. For 1999 JV6, we collected optical photometry between 2007 and 2016. Additionally, we obtained radar echo-power spectra and imaging observations with Arecibo and Goldstone planetary radar facilities in 2015, 2016, and 2017. We combined our data with published optical photometry to develop a robust physical model.
Results. We determine that the rotation pole resides at negative latitudes in an area with a 5 radius close to the south ecliptic pole.
The refined sidereal rotation period is 6:5367870:000007 h. The radar images are best reproduced with a bilobed shape model. Both
lobes of 1999 JV6 can be represented as oblate ellipsoids with a smaller, more spherical component resting at the end of a larger,
more elongated component. While contact binaries appear to be abundant in the near-Earth population, there are only a few published
shape models for asteroids in this particular configuration. By combining the radar-derived shape model with optical light curves we
determine a constant-period solution that fits all available data well. Using light-curve data alone we determine an upper limit for
YORP of 8.5 x 10-8 rad day-2.
Conclusions. The bifurcated shape of 1999 JV6 might be a result of two ellipsoidal components gently merging with each other, or
a deformation of a rubble pile with a weak-tensile-strength core due to spin-up. The physical model of 1999 JV6 presented here will
enable future studies of contact binary asteroid formation and evolution.