By Robin Respaut

June 7 (Reuters) - The California State Teachers’ Retirement System board voted unanimously on Wednesday to divest from non-U.S. thermal coal, affecting a very small fraction of the public pension fund's portfolio.

The fund estimates that $8.3 million of its roughly $206.5 billion portfolio is exposed to non-U.S. thermal coal.

The exposure is invested in three companies - PT Adaro Energy in Indonesia, Exxaro Resources Limited of South Africa, and Whitehaven Coal Limited of Australia.

"This is a serious decision," said California State Controller Betty Yee, who is a member of the CalSTRS board. But Yee noted that engagement with corporations is much harder when the companies are headquartered abroad, so CalSTRS engagement with non-U.S. thermal coal companies would be less likely.

Board member Tom Unterman said the vote to divest did not impact the fund in any way.

CalSTRS, the nation's second largest public pension fund, manages the retirement benefits of 914,000 California public school educators.

(Reporting by Robin Respaut; Editing by Leslie Adler)