  Tidal Dwarf Galaxies (TDGs) are objects presently forming from gas which has
been expelled from their parent galaxies during an interaction. We observed the
CO emission of a sample of 11 TDGs, of which 8 were detected. The CO is found
at the peak of the HI observations and has the same line velocity and width,
indicating that the molecular gas is forming in situ instead of being torn from
the parent galaxies. The presence of H$\alpha$ emission furthermore shows that
stars are forming from this molecular gas. In order to investigate star
formation in TDGs further, we compared their molecular gas content and star
formation rate (SFR), traced by H$\alpha$, to those of spiral galaxies and
classical dwarfs. The major difference between TDGs and classical dwarfs is the
lower metallicity of the latter. The star formation efficiency (SFR per
molecular gas mass) of TDGs lies in the range typical of spiral galaxies
indicating that star formation is proceeding in a normal fashion from molecular
gas.
