As taken from Bhrigu Samhita as written by sage Bhrigu Ketu rules Mithun Rashi (Gemini Sign) & Rahu is Rules Kanya Rashi (Virgo Sign).
Both are linked with Mercury, as both these signs are also ruled by Mercury
Rahu & Ketu exalted & debilitated
Further this chart also tells us about which star signs causes Rahu & Ketu to become exalted & debilitated.
As seen above, Rahu becomes exalted in Mithun Rashi (Gemini Sign) & Ketu becomes exalted in Dhanu Rashi (Saggitarius sign).
Further, Rahu becomes debilitated in Dhanu Rashi (Saggitarius sign) & Ketu becomes debilitated in Mithun Rashi (Gemini Sign) it's own sign.
