IT WILL BE MONTHS, IF NOt years, before it becomes clear whether a Royal Bank of Scotland–led consortium secured a good deal with its record €71.1 billion ($104 billion) takeover of ABN Amro Bank in October. But the acquisition has already created a juicy opportunity for canny U.S. investors.

Fortis, the Belgian-Dutch insurance and banking group that is taking ABN Amro’s domestic Dutch banking business as part of the deal, financed its share of the takeover by making a €13 billion rights offering. The September issue created a quick windfall for buyers of Fortis’s American depositary receipts because of a pricing anomaly between those ADRs and the group’s Brussels- and Amsterdam-listed ordinary shares. What’s more, the growing number of rights issues by international companies can offer similar arbitrage possibilities to investors, fund managers say.

Ed Cofrancesco,...