Petosiris and Nechepso are authors who are often quoted by ancient writers on astrology. Although they have typical Egyptian names, they seem to be pseudonyms rather than any identifiable individual. Only fragments of their books have survived, but there is one document, framed as a letter from Petosiris to Nechepso, which survived intact throughout the Middle Ages in a Latin version. Copies of the original Greek text of the document have also been discovered.
All the fragments of Nechepso and Petosiris have been translated into English by L. László, and his translation can be found in academia.edu. The letter appears there as Fragments 100-107. Also available online are standalone versions of fragments 99-102 and fragments 103-108. The translation includes diagrams of the so-called Petosiris Circle, a method of divination using numerology. The Latin and Greek texts of the letter can be found as Fragments 37-38 in the edition of the fragments by E. Riess ( Google Books ).
The writings attributed to Petosiris and Nechepso are generally dated to the late Hellenistic period, or possibly the first century A.D.