From A Treasury of Knitting Patterns* by Barbara Walker.
In this version of Double Brioche, you see Barbara Walker's original instructions: yo, sl1
on one row, slip the yarn over and knit the slipped stitch on the following row, then knit them together on the third row.
See also this Ravelry discussion.
*Affiliate link. Buy the book and I get a couple pennies.
Shown with a cast-on count of 6 stitches.