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 \lesssim ( M _ { 2 } / M _ { 1 } ) \lesssim 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 \sim 10 ^ { 6 } L _ { \odot } followed by a slow decline at very low effective temperatures .