Self-linking number of a real algebraic link
  For a nonsingular real algebraic curve in the 3-dimensional projective space and sphere a new numeric characteristic is introduced. It takes integer values, is invariant under rigid isotopy, multiplied by -1 under mirror reflection. In a sense it is a Vassiliev invariant of degree 1 and a counter-part of a link diagram writhe. 
Oleg Viro