  We derive the constraints on the mass ratio for a binary system to merge in a
violent process. We find that the secondary to primary stellar mass ratio
should be ~0.003 < (M_2/M_1) < ~0.15. A more massive secondary star will keep
the primary stellar envelope in synchronized rotation with the orbital motion
until merger occurs. This implies a very small relative velocity between the
secondary star and the primary stellar envelope at the moment of merger, and
therefore very weak shock waves, and low flash luminosity. A too low mass
secondary will release small amount of energy, and will expel small amount of
mass, which is unable to form an inflated envelope. It can however produce a
quite luminous but short flash when colliding with a low mass main sequence
star.
  Violent and luminous mergers, which we term mergebursts, can be observed as
V838 Monocerotis type events, where a star undergoes a fast brightening lasting
days to months, with a peak luminosity of up to ~10^6 Lo followed by a slow
decline at very low effective temperatures.
