It was a very sharp vapour trail, and very interesting to watch. At the very least, it showed the aerodynamics of the wing. And the sun made the outer area of the trail kind of glowing:
The phenomena disappeared after about 30 seconds: so not to worry about loosing fuel ;-). But what was it? After asking around, finally a got the answer: It is generated by the vortex generators on the engine. If the atmospheric humidity and dew point are right, such trails can be observed. Explanation and more about this cool phenomena is in “Thin Vapour Trails Above Wing“. Aerodynamics can be sooooo cool.
Happy Trailing 🙂