  Aims. To explore the chemical pattern of early-type stars with planets,
searching for a possible signature of planet formation. In particular, we study
a likely relation between the lambda Bootis chemical pattern and the presence
of giant planets. Methods. We performed a detailed abundance determination in a
sample of early-type stars with and without planets via spectral synthesis.
Results. We compared the chemical pattern of the stars in our sample (13 stars
with planets and 24 stars without detected planets) with those of lambda Bootis
and other chemically peculiar stars. We have found four lambda Bootis stars in
our sample, two of which present planets and circumstellar disks (HR 8799 and
HD 169142) and one without planets detected (HD 110058). We have also
identified the first lambda Bootis star orbited by a brown dwarf (zeta Del).
This interesting pair lambda Bootis star + brown dwarf could help to test
stellar formation scenarios. We found no unique chemical pattern for the group
of early-type stars bearing giant planets. However, our results support, in
principle, a suggested scenario in which giant planets orbiting
pre-main-sequence stars possibly block the dust of the disk and result in a
lambda Bootis-like pattern. On the other hand, we do not find a lambda Bootis
pattern in different hot-Jupiter planet host stars, which do not support the
idea of possible accretion from the winds of hot-Jupiters, recently proposed in
the literature. Then, other mechanisms should account for the presence of the
lambda Bootis pattern between main-sequence stars. Finally, we suggest that the
formation of planets around lambda Bootis stars such as HR 8799 and HD 169142
is also possible through the core accretion process and not only gravitational
instability [abridged]
