  Using high-quality spectra of the twin stars in the XO-2 binary system, we
have detected significant differences in the chemical composition of their
photospheres. The differences correlate strongly with the elements' dust
condensation temperature. In XO-2N, volatiles are enhanced by about 0.015 dex
and refractories are overabundant by up to 0.090 dex. On average, our error bar
in relative abundance is 0.012 dex. We present an early metal-depletion
scenario in which the formation of the gas giant planets known to exist around
these stars is responsible for a 0.015 dex offset in the abundances of all
elements while 20 M_Earth of non-detected rocky objects that formed around
XO-2S explain the additional refractory-element difference. An alternative
explanation involves the late accretion of at least 20 M_Earth of planet-like
material by XO-2N, allegedly as a result of the migration of the hot Jupiter
detected around that star. Dust cleansing by a nearby hot star as well as age
or Galactic birthplace effects can be ruled out as valid explanations for this
phenomenon.
