Rahu: In Indian Astrology Rahu is a shadow planet which is evil, tamsik and unfortunate planet in nature. Rahu always moves retrograde and is friendly with Saturn, Mercury and Venus. It is known as Dragon’s Head in Astrology.
Rahu was the name of Trijata ( 3 plaited monstrous) hence 3rd sign i.e Gemini was the exalted sign of Rahu.Adding 6 signs it is Sagittarius, hence Sagittarius is the sign of debilitation for Rahu is exalted in Gemini up to 15 degrees and debilitated up to 15 degrees in Sagittarius.
Ketu: Ketu is the shadow planet and is considered as the shadow of planet Mars. It is always positioned 7th from the Rahu as it remains 180 degrees from Rahu. This planet is considered to be the karak of Moksha and is the planet of sudden happenings and incidents. Ketu is always retrograde same as Rahu.
Since ketu is placed 7th from Rahu hence the exaltation sign of Rahu is the debilitated sign of Ketu and the debilitated sign of Rahu is the exalted place of Ketu.
Ketu is Exalted in Sagittarius upto 15 degrees and is debilitated in Taurus up to 15 degrees.