In this paper we have identified a new category of astrometric binaries based on
the peculiarities of the detection chain used by Hipparcos during the three years of
its observing mission. It appears that for pairs with orbital period less than 20
years and with angular separation larger than 
, it is possible to derive
the mass ratio between the two components and their magnitude
difference at the same time as the astrometric parameters of the centre of mass.
An extensive simulation, partly based on the Hipparcos observations, has allowed to
locate more accurately in the separation-period-magnitude space various regions
where the processing might end up with the two physical parameters or only with their
difference, as for the more common astrometric binaries. In some favorable
situations an accuracy of few hundreths in the mass ratio 
can be achieved. Finally we have identified two sets of about 20
stars each, all belonging to the Hipparcos program and having fairly reliable orbits,
which will be studied soon after the release of the Hipparcos catalogue and the
availability of the intermediate data.