  The frequency of planets in binaries is an important issue in the field of
extrasolar planet studies, because of its relevance in estimating of the global
planet population of our Galaxy and the clues it can give to our understanding
of planet formation and evolution. However, only preliminary estimates are
available in the literature. We analyze and compare the frequency of planets in
multiple systems to the frequency of planets orbiting single stars. We also try
to highlight possible connections between the frequency of planets and the
orbital parameters of the binaries (such as the periastron and mass ratio.) A
literature search was performed for binaries and multiple systems among the
stars of the sample with uniform planet detectability defined by Fischer &
Valenti (2005), and 202 of the 850 stars of the sample turned out to be
binaries, allowing a statistical comparison of the frequency of planets in
binaries and single stars and a study of the run of the planet frequency as a
function of the binary separation. We found that the global frequency of
planets in the binaries of the sample is not statistically different from that
of planets in single stars. Even conservatively taking the probable
incompleteness of binary detection in our sample into account, we estimate that
the frequency of planets in binaries can be no more than a factor of three
lower than that of planets in single stars. There is no significant dependence
of planet frequency on the binary separation, except for a lower value of
frequency for close binaries. However, this is probably not as low as required
to explain the presence of planets in close binaries only as the result of
modifications of the binary orbit after the planet formation.
