Do we have any idea what occurs if a young star and an old star merge? Of course my first thought on that line of thinking is that would cause some type of explosion or ejecta that would be noticeable, but maybe not.
In the case of slowly decaying orbits, one star usually ends up stripping most of the atmosphere from its partner, before the core eventually passes its Roche limit, breaks apart, and is accreted. In that case you get a series of Novas (not to be confused with supernovas), and a somewhat larger star when all is finally said and done. Lots of debris, occasionally very bright.
It's hard to imagine a situation in which two stars "merge" without a slow process of cannibalization. Compact collapsed bodies like white dwarves, neutron stars and the rest definitely do merge, but they're so much denser than a "living" star.