Three of the Big Four accounting firms in the US now have women in the corner office, but auditing still has a large gender gap.

A forthcoming study suggests women are underrepresented among the accounting-firm partners who head the outside audits of America’s biggest public companies. Only 15% of the “engagement partners” in charge of each S&P 500 company’s audit are women, according to the study by CFA Institute, which represents chartered financial analysts. The study is expected to be published this week.