Superscript open circle notation in formula I may be only a physicist but I'm familiar with the superscripted dot to indicate a radical.  In a paper I'm reading the authors have also used a superscripted open circle.  I can't find any reference online or in my physical chemistry textbook (Atkins). Here's an example:



As you can see they've done this a few times, and also used the dot to indicate a radical.  This is in a published paper "Photocatalytic degradation pathway of methylene blue in water", Houas et al., Applied Catalysis B: Environmental (2001).  I'm starting to wonder (given the OH◦ in the text and OH• in the equation) if it's a typo, but I'd be surprised if something so glaring could get through review or even proofreading by coauthors.

So does the open circle have a meaning of its own?
According to The Manual of Scientific Style: A Guide for Authors, Editors, and Researchers 9.2.5.12


  Indicate a free radical by placing either a centered dot or a superscript dot, signifying the unshared electron, next to the chemical symbol or compound formula


This convention is also followed by the ACS. 

So it would be better if the article used the solid symbol throughout, but the solid and open symbols mean the same thing in the article.