  Context. The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis
(ROSINA) was designed to measure the composition of the gas in the coma of
comet 67P/Churyumov-Gerasimenko, the target of the European Space Agency's
Rosetta mission. In addition to the volatiles, ROSINA measured refractories
sputtered off the comet by the interaction of solar wind protons with the
surface of the comet.
  Aims. The origin of different solar system materials is still heavily
debated. Isotopic ratios can be used to distinguish between different
reservoirs and investigate processes occurring during the formation of the
solar system.
  Methods. ROSINA consisted of two mass spectrometers and a pressure sensor. In
the ROSINA Double Focusing Mass Spectrometer (DFMS), the neutral gas of
cometary origin was ionized and then deflected in an electric and a magnetic
field that separated the ions based on their mass-to-charge ratio. The DFMS had
a high mass resolution, dynamic range, and sensitivity that allowed detection
of rare species and the known major volatiles.
  Results. We measured the relative abundance of all three stable silicon
isotopes with the ROSINA instrument on board the Rosetta spacecraft.
Furthermore, we measured $^{13}$C/$^{12}$C in C$_2$H$_4$, C$_2$H$_5$, and CO.
The DFMS in situ measurements indicate that the average silicon isotopic
composition shows depletion in the heavy isotopes $^{29}$Si and $^{30}$Si with
respect to $^{28}$Si and solar abundances, while $^{13}$C to $^{12}$C is
analytically indistinguishable from bulk planetary and meteorite compositions.
Although the origin of the deficiency of the heavy silicon isotopes cannot be
explained unambiguously, we discuss mechanisms that could have contributed to
the measured depletion of the isotopes $^{29}$Si and $^{30}$Si.
