First detection of the deuterium Balmer series

The first detection and identification of deuterium Balmer lines are reported in five H II regions of the Galaxy and the Small Magellanic Cloud. The D I lines appear as faint, narrow emission features in the blue wings of the H I Balmer lines and can be distinguished from high-velocity H I emission. The identification with deuterium and the excitation mechanism by fluorescence are both established beyond doubt. Measurements of deuterium Balmer series might lead to a new, optical method of deuterium abundance measurement in the interstellar medium and beyond.