The U.S. SEC voted yesterday to propose new rules for special purpose acquisition companies (SPACs), casting a big shadow over the booming market. If adopted, these public shell companies — formed to acquire a business and take it public without the fuss of the traditional IPO process — would have to provide more investor disclosures, especially about their ownership and performance forecasts. In some cases, SPACs may have to register as investment companies, subjecting them to stricter rules.