Rahu and Ketu have unpredictable nature hence what both shadowy planets will do in birth chart is also unpredictable. The information available in astrology books about Rahu-ketu is largely confusing. Sage Parashar and many others believe that their sign of exaltation is Taurus and Scorpio respectively, while many others, mostly modern astrologers think that Gemini and Sagittarius is their exaltation sign.
They find mention first in Atharvaveda 19.9.10 –

शं नो॒ ग्रहा॑श्चान्द्रम॒साः शमा॑दि॒त्यश्च॑ राहु॒णा। शं नो॑ मृ॒त्युर्धू॒मके॑तुः॒ शं रु॒द्रास्ति॒ग्मते॑जसः ॥

However this reference is not astrological at all, as in this Richa Ketu is Comet and Rudra is 11 Rudras who were considered inauspicious . Rahu finds mention in Mahabharata ( dating of text is confusing as many believe that slokas and chapters has been added from time to time. Like Purana it has been subject of regular editing. ). In Mahabharata it is said that Rahu is circular like Moon, its Diameter is 12 thousand yojana and circumference is 36 thousand yojana.
योजनानां सहस्राणि विष्कम्भो द्वादशास्य वै ।
परिणाहेन षट्त्रिं शद् विपुलत्वेन चानघ ॥ 
Ketu is tail of Rahu or South node of Earth is known in 5th century even. In old Samhita texts and also in Varahamihir’s texts we find him discussing comets of various forms and color instead of South node Ketu. Since fifth century onwards many great astrologers did research for few more centuries and found that they Rahu-Ketu have no character of their own and behave like an agent. They are said to be auspicious in few sign and houses and mostly inauspicious in other. Rahu’s character is similar to Shani “shanivat Rahu) and Ketu’s character is like Mars (mangalvat ketu). They are said to be Rajayoga karak in Angular houses mostly in 10th house and Trikon houses 1, 5, 9, in upachaya houses also they are said to be very good.
But here are an interesting sutras of Kalidas regarding 10th and 9th house –
1- If Rahu or Ketu are in 9th and conjunct with 10th lord or Rahu-Ketu are in 10th house and conjunct with 9th house lord or Rahu-ketu are in 9th house and lord of 9th is in 10th house or vice versa, In all these conditions they become Yoga karaka and give auspicious result in their dasha period. If they are even any how related to the lords of 10th and 9th then also they become Yoga karaka.

2- If Rahu or Ketu are in 9th and conjunct with 9th lord or If Rahu or Ketu are in 10th and conjunct with 10th lord then they become very auspicious and Yoga karaka. But if the lord these house are the owner of 8th or 12th house then their auspiciousness is destroyed and they will produce evil effect in their daha period.

These Yogas should be judged properly in lagan and navansha chart both before doing any prediction. Any affliction by aspect and association with lord of bad houses of may destroy the yoga.