An Assumption

In the Algol paradox of the left frame, we have assumed that the two stars of the binary were formed together and are the same age. It is also possible that they did not form together and were later captured into orbit around each other. Then it need not be a paradox for the more massive star to be less evolved.

However, we believe that most binaries were formed together because capture is a highly unlikely process. Therefore, for most Algol-like systems there is a genuine paradox that can be resolved by the mass transfer mechanism discussed in the left panel.