  Ultra-short orbital period contact binaries (Porb < 0.26 d) host some of the
smallest and least massive stars. These systems are faint and rare, and it is
believed that they have reached a contact configuration after several Gyrs of
evolution via angular momentum loss, mass transfer and mass loss through
stellar wind processes. This study is conducted in the frame of Contact
Binaries Towards Merging (CoBiToM) Project and presents the results from light
curve and orbital analysis of 30 ultra-short orbital period contact binaries,
with the aim to investigate the possibility of them being red nova progenitors,
eventually producing merger events. Approximately half of the systems exhibit
orbital period modulations, as a result of mass transfer or mass loss
processes. Although they are in contact, their fill-out factor is low (less
than 30 per cent), while their mass ratio is larger than the one in longer
period contact binaries. The present study investigates the orbital stability
of these systems and examines their physical and orbital parameters in
comparison to those of the entire sample of known and well-studied contact
binaries, based on combined spectroscopic and photometric analysis. It is found
that ultra-short orbital period contact binaries have very stable orbits, while
very often additional components are gravitationally bound in wide orbits
around the central binary system. We confirmed that the evolution of such
systems is very slow, which explains why the components of ultra-short orbital
period systems are still Main Sequence stars after several Gyrs of evolution.
