The letters 'a', 'e', 'i', 'o', 'r', 't', and 'u' have already been guessed. Based on the current word pattern 'r__t__', the missing letters are likely to be consonants. The most common consonants in English words are 's', 'n', and 'l'. I will guess 's' as it is a common consonant that fits well with the context of the word.