  We observed with the SOPHIE spectrograph (OHP, France) the transit of the
HAT-P-6b exoplanet across its host star. The resulting stellar radial
velocities display the Rossiter-McLaughlin anomaly and reveal a retrograde
orbit: the planetary orbital spin and the stellar rotational spin point towards
approximately opposite directions. A fit to the anomaly measures a
sky-projected angle lambda = 166 +/- 10 degrees between these two spin axes.
All seven known retrograde planets are hot jupiters with masses M_p < 3 M_Jup.
About two thirds of the planets in this mass range however are prograde and
aligned (lambda ~ 0). By contrast, most of the more massive planets (M_p > 4
M_Jup) are prograde but misaligned. Different mechanisms may therefore be
responsible for planetary obliquities above and below ~3.5 M_Jup.
