I wouldn't call it research, but we had two cats for sixteen years, half a year ago one of them died. The one left seems now unhappy without a companion - he "speaks" a lot - before he wasn't very noisy if at all. He expects us to be with him much more often than before and seems unhappy when we do our own things, although we care for him as much as before or even a little more.
When there were two cats, they often slept together - now the one cat tries to sleep on us every night. They washed each other - especially the heads and ears - no human can do that :(I sadly have to admit. They played together too.
From my observation a companion is a good thing for a cat (if you can afford it - because it is double the cost of the food, sand and the vet x2). But I'd think about getting a young cat, because for two old ones, becoming friends could sometimes be a long and a hard way.