For cots, you need to determine the fruiting habit of the variety that you have. Some produce near branch ends, some produce in the middle section of branches and some at the third of the branch close to the branching point. For the first two types, it is best to do major pruning in the summer (this stimulates production of fruiting wood) just after harvest and then some thinning cuts (for air/sun penetration) late winter/early spring. For the third type, I would do the pruning either like the first two or late winter/early spring. For plums, I would prune them like the third type. For me, late ripening plums are always pruned in late winter/early spring (as there is not enough time left in the season to produce new fruiting wood). I always follow my winter/spring pruning with a copper/sticker spray.
See below too 
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