Telepathic Dialogue Punctuation
You may be looking for /, the symbol for "slash".
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Using // around telepathic dialogue in fan fic is a method of distinguishing telepathic discourse.
// was a much more popular device in early Star Trek fiction -- 1960s through the 1970s -- than it is today. Telepathic dialogue in Science Fiction of that period used italics or, as in Robert Heinlein's Stranger in a Strange Land, ("italicized parenthetical quotes").
Since fanzines were produced on typewriters, the use of italics was impractical. (This is possibly why Stephen King used simple (parentheses with all lower-case lettering) for telepathy and private thoughts.) The use of // receded when writers began using personal computers where text could easily be set in italics.
The First StoryLelamarie S. Kreidler, the author of "Time Enough" had this explanation at the beginning of her story:
The time of this story is the future indefinite. The girl involved is, like Spock, a Terran-Vulcan hybrid, although her mother was of Vulcan and her father was a former Star Fleet officer living on Vulcan. To indicate communication from mind to mind, which is common among Vulcans, I have used the slash symbols instead of quotation marks. Thus, //...//, instead of "...", to show that the dialogue is not spoken aloud. L.S.K.
Later UseIn Boldly Writing, Joan Verba wrote:
Years later, Jean Lorrah, in her first solo professional novel, also used such marks to distinguish spoken dialogue from telepathic dialogue. When I asked her why she used this indicator without explanation, saying that readers of her books who were not also fanzine readers might not understand this, she replied it was a commonly-known indicator, and that she thought no explanation was necessary.
Not all fans were fans of the "//". A fan in 1986 wrote: "Thank you for NOT using "//" to indicate thoughts— I've run across that in some other zines lately & it is very annoying." 
a page from Time Enough (April 1969)
an example from More Missions, More Myths #5 (November 1986)
a page from A House of Cedar, a Kirk/Spock story (July 2002)