A call option on a stock is a common and widely used derivative. On an average trading day in 2015, more than 800,000 such options traded on the Chicago Board Options Exchange, the largest options exchange in the United States. Each option grants its owner the right to buy 100 of a specific...

A classic result by Merton (1973) is that, except just before expiration or dividend payments, one should never exercise a call option and never convert a convertible bond. We show theoretically that this result is overturned when investors face frictions. Early option exercise can be optimal...