  Since 2008 we used high-precision radial velocity (RV) measurements obtained
with different telescopes to detect signatures of massive planets around
main-sequence and evolved stars of the open cluster (OC) M67. We aimed to
perform a long-term study on giant planet formation in open clusters and
determine how this formation depends on stellar mass and chemical composition.
A new hot Jupiter (HJ) around the main-sequence star YBP401 is reported in this
work. An update of the RV measurements for the two HJ host-stars YBP1194 and
YBP1514 is also discussed. Our sample of 66 main-sequence and turnoff stars
includes 3 HJs, which indicates a high rate of HJs in this cluster (~5.6% for
single stars and ~4.5% for the full sample ). This rate is much higher than
what has been discovered in the field, either with RV surveys or by transits.
High metallicity is not a cause for the excess of HJs in M67, nor can the
excess be attributed to high stellar masses. When combining this rate with the
non-zero eccentricity of the orbits, our results are qualitatively consistent
with a HJ formation scenario dominated by strong encounters with other stars or
binary companions and subsequent planet-planet scattering, as predicted by
N-body simulations.
