  It has recently been proposed that the dwarf spheroidal galaxies located in
the Local Group disks of satellites (DoSs) may be tidal dwarf galaxies (TDGs)
born in a major merger at least 5 Gyr ago. Whether TDGs can live that long is
still poorly constrained by observations. As part of deep optical and HI
surveys with the CFHT MegaCam camera and Westerbork Synthesis Radio Telescope
made within the ATLAS3D project, and follow-up spectroscopic observations with
the Gemini-North telescope, we have discovered old TDG candidates around
several early-type galaxies. At least one of them has an oxygen abundance close
to solar, as expected for a tidal origin. This confirmed pre-enriched object is
located within the gigantic, but very low surface brightness, tidal tail that
emanates from the elliptical galaxy, NGC 5557. An age of 4 Gyr estimated from
its SED fitting makes it the oldest securely identified TDG ever found so far.
We investigated the structural and gaseous properties of the TDG and of a
companion located in the same collisional debris, and thus most likely of tidal
origin as well. Despite several Gyr of evolution close to their parent
galaxies, they kept a large gas reservoir. Their central surface brightness is
low and their effective radius much larger than that of typical dwarf galaxies
of the same mass. This possibly provides us with criteria to identify tidal
objects which can be more easily checked than the traditional ones requiring
deep spectroscopic observations. In view of the above, we discuss the survival
time of TDGs and question the tidal origin of the DoSs.
